From my experience, all fruit must be cooked and mashed up to feed babies. A recent medical study has adviced that babies should have first been introduced to solid food at 6 months of age rather than 4 months. If your doctor says 4 months then he is very likely not a up-to-date practice. you should telephone BC Nurse line, it is 24 hours service, you can ask for a Mandarin service, they will assist you with all you need.
If your baby continue to have diarrhea for more than a few days, you should take him/her to see your doctor. Your baby is now 6 weeks old, if your breast milk is intorelatble to your baby, the Nurse or Doctor will be able to tell you from her/his growing chart.
Remember, try not to be your baby's own doctor. Helps is everywhere.