I recently found little red bumps on my cats neck under her collar. What could these be?

Those red bumps could be just about anything, from an infection to an allergic reaction to an irritation from her collar to fight wounds.  If the red bumps are ONLY under the collar, you might want to remove the collar and see how she does for several days without it on.  If the bumps are more spread out, then other more involved causes will need to be considered.

Helpful Buckeye

Absolutely, the fleas could very well be the cause of the red bumps.  That would be the allergic reaction I mentioned in my first response.  When a flea bites a cat or dog, the flea saliva that gets into the bite wound can cause an allergic reaction that leads to redness and sometimes intense scratching. 

Controlling the fleas might totally eliminate the red bumps you are seeing.  Since she is only 4 months old, be careful what you use for flea control.  Be sure to use a product that is labeled for use on a young kitten.

Helpful Buckeye