Once you have a 4 channel scope you'll never go back to a 2 channel scope. Its true you won't be using 4 channels all the time but I find myself using more than 2 channels quite often.
I would not like to have a lab that only has a 2 channel scope.
If you can afford a 4 channel scope, get one.
It's like having a deep memory scope. You don't need it all the time, but it's good to know you have it so you can go "Hey, I can do that, I'll jut switch on deep memory".
With a 4 channel scope you can go "Hey, I can view that at the same time, I've got 4 channels!"
You might not think you need 4 channels if you have always been limited to 2 channels and you had no choice.
The Rigol DS1054Z, the defacto standard entry level scope, has now made 4 channels kind of an expected thing.
There are countless usage scenarios. One might be where you are debugging a circuit. It's common to have one probe on the rail to make sure there is no funny business there (sometimes more than one rail), one for the signal you are inspecting, and another as a trigger signal you also want to view and not just trigger from, maybe another on the reset line looking for funny business there etc.
Basically, 4 channels gives you the ability to hunt for those time correlated "funny business" problems.