A four month old kitten being pregnant is highly doubtful. Not impossible. Just doubtful. IMO IF the kitten were pregnant she would be aborted faster than a trip to the litter box. If the kitten is literally just over four months old that would mean she mated at around 3.5 months or younger in order to be showing now.
Do you know when she was dewormed, what was used, and how often? A cat needs to be wormed more than once to ensure all of the worms are gone. Also, depending on what meds were used, they may not have killed all of the worms. There are many types of worms which react differently to different meds. Some meds will only work on tape worms. Some will only work on round worms. Not to mention hook and lung worms.
I'm glad she is going to the vet to be checked out.