HI R.,
I started all 3 of my kids on cereal at 6 months (especially since they were all born early). All 3 of my kids just seemed too young, mentally and physically, to take on cereals at 4 months. I suppose that some kids are ok with it at 4 months, but I just didn't want to chance it with mine. Maybe you should wait until 6 months too. By 6 months, baby will also have more teeth.
When you begin again, start slowly; use the same cereal for about 4 days to make sure there are no allergies, then switch off to another cereal for another 4 days. Once you've established that baby is not alergic, then you can begin combining or just giving both (or more) throughout the day. This is the same technique I used when it came time to feed veggies and fruit. I stuck with the same fruit or veggie for 4 days straight, then switched to a new fruit, once there wasn't any reaction to the previous. After about 2 weeks baby can eat about 4 different fruits and veggies.
Lastly, rule of thumb...begin with veggies ONLY for all of your preliminary 4-day-test-feedings. If you begin with fruit first, baby may not take veggies as easily, or at all.
Good luck!
Mari