Basement Rough-in Identification
1 is a spare stack and it's 3" abs

2 is the toilet also 3"

3 is a vent and

4 is a sink.

It's a pretty bad rough in. If 3 is in fact a vent why is it so far away from the wall?
If it's for a shower/tub why is it so close to the wall?

This is what your rough in should look like

3 must be a vent. 

EDIT
This is an example of what is going on
www.doityouself.com

If you can pour water down one of the 2-inch pipes and see that it holds water at level, that would indicate it has a trap as in the pipe at the far left of the image. Traps it only ever be installed at floor drains or shower drains it sink drain will not have a trap under the floor. 

Your 3-inch that's close to the wall may be repurposed as a shower drain provided it's vented. 

In your image pipe number 3 could very well be intended as a shower drain, just in a very poor location.