You may wanna read up on this at Is Baby Ready for Solid Food? Signs of being ready for solid foods at wholesomebabyfood.com. Take a look at the list of cues and see if your baby is ready.
In any case, the earliest to start is 4 months, not any earlier. You may wanna read this too for early introduction of solids.
If you really think your baby is ready at 4 months, go ahead and start. Otherwise, if your baby is gaining wt well on milk alone so far, it is advisable to hold back till baby is about 6 months.