Originally Posted by Frankie

No, the 4104 did not come stock with an effects loop, so those two jacks you have on the back are your aftermarket loop I assume. I can't say for sure without seeing the guts, but you could plug something in and find out. It's not gonna blow up if you plug a pedal in backwards or anything, it just won't make sound.

Marshall Amps Info & Schematics

Scroll down to the 4104 or the 2204. You can get a user manual and schematic there for your amp.