I came home after being away for 4 days and found a dead mouse in my toilet.

There are no mouse droppings in the house. Could the mouse have come through the pipes/sewer?

Possible.

Possible, for sure! Also as likely is the mouse getting a drink of water and drowning. I'd get a few mouse traps just to be sure... ;)

Any cats in the home?

Mark

Anything is possible, there is just no way to tell.