Edit: Your ped almost certainly will say it's okay to start solids. Most do, as I indicated below. Just realize that, while it's not a terrible and dangerous thing (hence most peds will still say it's okay, even though they go against almost every health organization in doing so), it's not the best thing for your child.
I agree with the posters who say none. 4 mos is too early for solids. The World Health Organization, American Association of Pediatrics, American Academy of Family Physicians, and a whole bunch of other health organizations recommend waiting until at LEAST 6 months. 4 mos was the old recommendation, and some doctors will still tell you that you can start solids then, but research has shown it's healthier to wait.
Introducing solids before 6 months is linked to higher rates of obesity, type 1 diabetes, asthma and allergies (I believe there are other things, but I cannot remember them all off the top of my head). Infants' GI tracts are also not really ready for solids before at least 6 mos, and in addition to causing GI discomfort, solids create tiny little perforations in the GI tract that make the baby more susceptible to illness.
Introducing solids at a particular age is sort of silly, anyway. Babies develop at different rates, so when my baby is ready for solids may be different than when you baby is ready for solids (just like they may roll over at different times, start cooing at different times, etc.). The signs of being ready for solids are: Sitting unassisted, doubling birth weight, losing the tongue-thrust reflex, opening mouth and leaning forward for spoon. When the baby has reached all of these, he/she is ready for solids. I don't know of any babies that actually lost their tongue-thrust reflex until at least 5 mos. There is no medical reason to introduce solids until after 6 mos, and even after 6 mos, solids are really for tasting and practice, not for nutrition. Babies' main source of nutrition should be breastmilk or formula until 1 year (this is why you should only give solids after a full "meal" of breastmilk or formula).
When your baby is ready for solids, we did rice cereal for a couple of weeks (the Earth's Best brown rice kind, since white rice is a pretty nutritionally-void simple carb) and then went on to organic oatmeal.