Alright I have an Engl Pre Amp running into a Mesa Boogie Simul 2-90 power amp, into a Mesa Boogie Slant Recto Cabinet.

Which input do I plug in to to use all 4 speakers? This might be a silly question but I don't know and all I've ever used until now is a 1x16 ohm 12 inch speaker into a laney head.

My power amp outputs are 8 Ohm Mono x1 and 4 Ohm Stereo x2 (two 4 ohm stereo outputs)

My cabinet coincidentally has two 4 ohm inputs that are split and an 8 ohm mono input.

So do I plug from the 8 ohm mono on the power amp into the matching 8 ohm mono in the cabinet, and if so does that use all 4 speakers?

I hooked this up last weekend at a friends show and I noticed it wasn't as loud as I thought it should have been, but now that I'm thinking back on it I think I might have plugged one of the 4 ohm speaker outputs into the 8 ohm speaker input on the cabinet.

Any clarity out there for this lost soul?
8 ohm mono in to 8 ohm mono.
double check if you feel the need by sticking you head in front of each speaker.
Shut up and play your guitar!