To be honest don't be a girls guest to a party she got invited to
The guys inviting her didn't need you there and were probably mildly surprised you and another male showed up.
It is very common for males to pay and not the females in Taiwan and also very common for the males to share the tab leaving the females free of charge.
My TW bud and I used to do that. We could bring a female guest or not but the tab was always split between us whether one had brought a female guest or both did or whatever. The ladies got away clean, no matter what they ordered.
So my thoughts are. DON'T go to an event that a FEMALE has been invited to by a MALE. Because YOU were NOT invited personally by the Male or Males in that party and therefore you should consider yourself UNwelcome.
It was right of you to ask about how much it would cost and you should NOT have gone without knowing a clear answer.
And yes you may have gotten lucky and not been presented with a bill if the other guys in the group were Gentlemen, but they appear not to be.
And yes 4000 a person is steep. If it was only 400 each of you, i guess you wouldn't even be writing about it here.
I know that I would NOT accept a ladies invite to join a group where there were males I didn't know and were not invited by.
I would invite said lady to another event that I set up instead.