I have a problem with my son who is almost 4 years and he still can’t use the toilet/potty for poop but he’s okay with peeing.

Your son may have a negative association with using the potty for a long call.  Try and use the main toilet instead for him.  Tell him that he is a big boy and offer rewards, kisses and hugs when he uses the toilet correctly.  Avoid scolding and complaints when he doesn’t use it successfully.