Classic_Spaceman

The ninja mask that is on the leaked "404" Minifig is actually Cole's, so he could wear any kind of headgear in his proper assembly.

Back on-topic: My prediction for the contents of the First Order Battlepack, based on LEGO's past releases. Recent Battlepacks (with some exceptions) generally employ a 2-1-1 format with regard to Minifigures (See spoiler). Thus, since the FO Battlepack is likely Sith Trooper-focused (based on its codename), the Minifig breakdown will probably be: Sith Trooper (2x), Sith Jet Trooper (1x), FO Stormtrooper (1x).

If that were the case, though, it would have been mentioned in the description. ? Probably the reason that the Blue Parker Minifig was not referenced, is that Parker's standard outfit and hair are already close enough in colour to Jack's ghostly blue look, such that whoever saw the sets simply did not notice the difference.

This: https://www.bricklink.com/v2/catalog/catalogitem.page?P=88811#T=C

Per the PROMOBRICKS description, the Hidden Side Portal includes four Minifigs - Three of which are Blue Jack and two red ghosts. Blue Parker would be a logical fourth Minifig.

I have a theory about "404": He is Mr. E. Their colour-scheme is the same (with the addition of blue in the case of "404"), "404" appears to have silver metal legs, and, aside from the three SoG Generals, no other Ninjago characters have numbers on their outfits (including the leaked Minifigs), so this is an odd stylistic choice for someone unrelated. Additionally, the 404 reference fits with the existing play on the word "mystery" - I.E. "mystery character not found". Story-wise, Mr. E is the only Nindroid whose creator is unknown (Harumi's comment that he was "built to be a cut above the rest" precludes the producers' original plan for him to be Echo Zane), so he could be connected to Unagami/Prime Empire. Furthermore, even if Mr. E were not originally associated with Unagami, he could easily have created a backup of himself at Borg Tower before he was 'killed' (which would not be the first time that this has occurred - See Zane), and somehow ended up in the game.

A few months ago, a White/GitD Parker head leaked (likely from the Hidden Side Portal set), so Jack's blue appearance is probably just how humans look when in the Hidden Side, rather than reflective of his being a ghost.