As others have said, the official line from the American Academy of Pediatrics is to wait until your baby has lost the tongue thrust reflex, can hold his head up, can sit, and is over 6 months old.
The first three are easy to assess, and the fourth is kind of arbitrary. Allergies occur when your immune system reacts aggressively to a benign substance. If your first exposure to a substance occurs when your immune system is immature, you're more likely to have an allergic reaction to it. The age at which a baby's immune system is sufficiently mature is unique to each baby, but is typically around 6 months. Of course you can't tell exactly when just by looking at your baby -- but you can consider your family's medical history.
If you or your husband have allergies, it would be sensible to wait until that six month milestone, or even later. If you and your husband can sit in a field of ragweed in the middle of a duststorm while petting an array of dogs and cats, then chances are that four months will be just fine.
Just use your common sense to figure out how ready your baby is. If allergies aren't a concern, and your 4-month-old is sitting up and looking at your plate and trying to get at the contents, then I'd say bring on the solids.