4 bishops or four knighits.

then you have two more rooks.

but another question what about castling in this four rook world.

If the best my opponent can do is play for a draw with passive defense then I'm not unhappy

Although 4 knights vs 4 rooks might be interesting. The 4 knights might get a big attack early when the rooks are still terrible.

Lasker would go for four knights...

But I would take up the four bishops.

bishops because it's easier to open a position than to close it.

there isn't a guarantee that here will be forks in the game if your opponent is half-decent. 

four knights are more fun though

the you cannot force your opponent to close the position in normal chess you can make it the best move to close the position but with four knights (unless tactics state otherwise) the non-four knight player will just not close the position and have an advantage

I'm all for knights

 once comment fifty has happened i'll look at an engine and see who's right.

knights is easier

I believe that four bishops is worse off than four knights. 

Extra bishop of same color is worth about 50%

Bishops and knight suffers heavily from redundancy in RNNQKNNR vs RBBQKBBR position.

For instance, which side do you prefer in the variant Chigorin Chess? 

https://en.m.wikipedia.org/wiki/Chigorin_Chess