I was wondering what the use is as well. I'm wondering if the courses of block and brick under those bearing points have cells grouted solid or not... It also looks like there is some minor water damage in that area as well. Do you plan on turning that into a closet or something useful?
What is the abandoned joist hanger for? Why does it look like the first joist to the right of the opening isn't attached to the floor beam (no hanger, can't see any toenailed framing nails)?
Looking at this, the 2x4 isn't doing anything. It literally isn't supporting the load of the floor above. The floor beam spanning that opening is doing all the work. You can cut that out and it won't affect anything. HOWEVER, based on the fact that it's bowing, there may be more of an issue than that sill plate. The floor beam spanning that opening is being overloaded and thus probably also bowing.
There are a few options you can do.
OPTION #1 - if you don't care about losing some of that space, or having access to the full 40" opening, you can get an adjustable steel column to and place it mid-span, to shore up and straighten the bow in the 2x4 sill. Then, add another 2x6 (or whatever the floor joist structure is) to span the opening, strengthening that area. Make sure you have a minimum of 6" of bearing on each end of that opening. If it doesn't bother you, leave the column in there. On the right side, take a 6x4 member, cut to the height of that brick wall, and place it at the right end, under the sill plate to allow for 5.25"(+/-) more of bearing at that end. Make sure the 6x4 is anchored to the wall, the floor and the sill, using steel straps.
OPTION #2 (better option) - Purchase two 4x6 to use for columns at each end of the opening. Build a header to span over the two columns using (2) 2x8. Attach the columns to the wall and floor and the header. (See attached sketch)
With either option, I would still attach another floor beam over that opening with a minimum 6" bearing each end.
As for bearing at an opening, typically, for a 40" opening, you need a minimum 6" bearing on each end of the opening .
Just FYI, I work in the structural engineering field. Currently employed in a full architecture/engineering firm in the structural engineering department.