Eva-04 is the fifth Evangelion completed, the third "Production Model" Evangelion. She and her sister unit, Eva-03, were constructed in the United States of America. Eva-04 was constructed at Nerv's second branch base (Nerv-2) in Nevada, while Eva-03 was constructed at Nerv's first branch in Massachusetts.
Eva-04 was used as a test subject for the restored S² Engine from the Fourth Angel, Shamshel. At the exact moment that Eva-04's S² Engine experiment was scheduled to begin, Nerv's entire second branch base, along with everything else in an 89 kilomter radius, simply vanished in a flash. Ritsuko Akagi and the Nerv technicians stated that it was not entirely clear what happened. One possibility was that all matter surrounding the base was instantly converted into energy, vaporizing everything. Another possibility is that the S² Engine accidentally opened up a Dirac Sea pocket dimension, similar to the one that the Angel Leliel sucked Eva-01 into, and Eva-04 along with the entire Nerv-2 base was sucked inside of this new Dirac Sea. Sabotage (probably by Seele) was considered a possibility, but there was nothing to overtly suggest this. Further, when Gendo Ikari and Kozo Fuyutsuki are speaking privately, they speak as if Seele reacted in shock to the Eva's disappearance and that it genuinely was an accident. In any event, it was never determined exactly what happened to Eva-03 and Nerv-2. The immediate result was that the United States government panicked at the loss of the base, and demanded that Eva-03 be transfered from their soil to Japan as soon as possible.
Evangelion Unit-04 never actually appeared on-screen at any time during the course of the series, however, production art released by GAINAX depicts Eva 04 as being physically identical to her sister unit, Eva-03. According to this material, the Eva was colored silver, with black and red detail. Eva-04 does make appearances in various Evangelion spinoffs such as video games, etc.
Purpose
Ostensibly, like all the Evangelions, Unit-04's stated purpose is to defend Tokyo-3 and the whole world from the Angel threat. It was also used as a testbed for NERV's attempts to create an artificial S² Engine for use in the Eva Series.
Abilities
Eva 04 never even physically appears on-screen, so any new abilities she may have been designed with are unknown. But had the production been a success, the Eva would've been able to operate without an umbilical cable.
Armor Design
Eva-04 follows the same armor layout as her sister unit Eva-03, predecessor Eva-02 and the repaired Eva-00. All Evangelions using this arrangement have five armor plates on their torso. The lowermost segment is elongated downwards to also cover the crotch, above this are two more segments, and all three wrap around the sides of the Eva's body to meet a vertical line of armor segments that cover the spine. The fourth segment from the bottom is the central chest plate, which is somewhat larger than the other segment, and instead of going around the back, terminates where the arm meets the bottom half of the shoulders.
Towards the end of this chest segment section on each side, near the shoulders, are small recessed cavities, painted an off-color from the majority of the armor. Above the central chest segment, the fifth and last armor segment is smaller than the rest and is essentially in the position of the collarbone. Above this, there is a throat-armor region which is not composed of segments like the rest and is colored purple in the Production Model (while light orange on Eva 01 and pink on the MP Evas). The throat region is covered with several sections of flexible material.
Soul
The soul of Shinji Ikari's mother, Yui Ikari, resides in Eva-01, and the soul of Asuka Langley Soryu's mother, Kyoko Zeppelin Soryu, resides in Eva-02. It is stated that Shinji's school class is actually a secret pool of potential Eva pilots, all grouped into one place. Also, it is gradually revealed or implied over the course of the series the mothers of all of Shinji's classmates are dead (Kensuke Aida directly states this, and Hikari Horaki and Toji also heavily imply this). Thus it seems to be implied that there is some connection between all of the potential pilots having "dead" mothers.
The pilot of Eva-04 is never specified. Shinji Ikari is the "Third Children" and when Eva Unit 03 was readied for use (after the loss of Eva-04) its pilot, Toji Suzuhara, was designated the "Fourth Children". Given that there was no designated child number between Shinji and Toji, it seems that in all probability she simply did not have a pilot assigned to it yet when it disappeared. For example, work on Eva-01's construction lasted for years before Shinji Ikari was assigned as its pilot.
Thus, it is heavily implied that the soul inside of each Eva is the soul of the pilot's mother (i.e., the soul in Eva-03 is probably that of Toji's mother), but it appears that a pilot was not yet selected for Eva-04, so it cannot be determined what soul was used in her creation. For that matter, it is not entirely clear at what point in the construction process the soul would be "inserted"; that is, if Toji was honestly "selected" as a pilot only after Nerv-2 disappeared, the soul of his mother would have been inserted into Eva-03 when its construction was almost fully complete. However, if Toji was always intended to be the pilot of Eva-03, years in advance, his mother's soul would already have been inserted into it years before (as was the case with Shinji and Asuka's mothers). Therefore, if Eva-04's pilot had honestly not been selected yet, the matching soul would also probably have not been inserted yet. But the other scenario is that one of Shinji's classmates was already selected, years in advance, to pilot Eva-04 and their mother's soul was already present inside of her.
Notes
Eva-04: the Eva that never was. Note that it should be silver, not whiteEva-04 was never actually seen on-screen, however Gainax has produced production sketches of it revealing that it was identical in design to Eva-03, but colored silver (that is, "chrome"), with red and black highlights. The only full body image they ever produced of Eva-04, however, mistakenly colored it white instead of silver: other close-up images of Eva-04 show it as silver, and in the numerous Evangelion spin-off merchandise it appears in such as video games and action figures, it is always silver.
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