If it has proper healed then you  can start gradually

Yes. She can start playing sports again, with gradual improvement in her core strength & aerobic activity. Stitches might have healed by now.

Start with Static exercise and then go for PRE exercise.

Make a protocol of exercise.  Do yoga starting with short duration and gradually increase time. If pain stiches site suddenly stop doing that exercise. Do pelvic floor muscle exercises and also make diet charts

core exercise

Can do the exercises  under the  guidance  of physiotherapist  stepbystep

First of all mostly weight reduce through proper balanced diet and breast feeding . Yes toning and strengthening of muscles will happen through proper exercises. She can definitely start exercising as its already 4 months but she has to start with basic abdominal back  and pelvic floor muscle strengthening and stretching exercises. She can also include walking in her regular regime . Once the strength start improving then she can make her exercise regine extensive. Yes of course with proper training and guidance she. can always come back to sports.

Yes, she can start Training for Sports .

It has been 4 months, so I dont think there will be any issue in returning to routine exercises.

She can start with static exercises in order to maintain tone of muscles.Focus on core muscle strengthening,pelvic floor exercises, flexibility exercises, strengthening of upper and lower extremity but all of the above should be done under supervision and after consulting your gynaec.