can i use blood for blood "after one of your (martell) characters is killed, choose and kill a character with equal or lower strength…" if im using quentyn as my agenda how does his replacement effect work with/against blood for blood

another question, can you trigger princes plans without having 4 cards in your discard pile? or do i have to have at least 4 cards to play it

Yes, Blood for Blood works when Quentyn is killed and becomes your agenda (he is still killed afterall, he just ends up in a different location than the dead pile)

Yes, you can play Princes Plans even with less than 4 cards in your discard pile.