Assuming all your ace and four outs are clean (e.g. villain doesn't have AJ or J4), you do indeed have 7 outs, but you've got two chances to hit them: the turn and the river. Using the rule of 4 and 2, seven outs twice has about 7*4 = 28% chance of hitting.
Equilab tells me that if villain has JTo, you have 27.8% equity.
The pot lays odd of 2600:200, but that means you only need to win 200/(2600+200) to break even. 200/2800 is about 7%, which is the proportion of the final pot you'd be contributing with this call.
It's basically a snap call, because the price you're getting is incredibly cheap (7%), and you've got more than enough chance of winning (28%) to make the call.
FWIW, I would love to play in a game where the button limp-calls from a 2.5bb stack, leaving half a blind for post-flop play.
You should probably read the
mathenoobics stuff in Beginner's Questions, because pot odds and outs are fairly basic, whereas this theory sub-forum is more advanced.