Still not 100% certain if I'm reading this card correctly. On my turn I play Banzai!. Do I have to decide immediately if I want to resolve the ability twice (hence +4 military instead of +2), or does it / can it take a completely different action later in the conflict?

In short, is this card only playable as ONE action, or does it take TWO actions if one decides to resolve it twice?

Action : During a conflict, choose a participating character – that character gets +2[Military] until the end of the conflict. You may lose 1 honor to resolve this ability twice. (Max 1 per conflict.)

It only takes one action.

The key is you decide to resolve it once or twice, not “you can do this action again”. So, if twice, then you just go to the part after the two dots and do it all over again, but as part of the same action. Which, BTW, means that you can choose another character and give him the +2. In fact, a twice resolved Banzai! it’s a +2 & +2, not a +4.