FWIW, as an adult with ADD (I got diagnosed 5 years ago at 31 - no mean feat, I can tell you - it's easier for adults now), I can tell you that stimulant meds (Ritalin, Concerta etc) work a treat, and actually make you feel better within yourself - you're not constantly looking for trouble (does that sound familiar?).
however, four is very young to make such a decision, and you'd have to be led by your consultant, as well as your own conscience and opinions, and those of your immediate family.
The stuff does work, and, in the case of Ritalin, almost immediately, but obviously, I took it as an adult - fully informed and willingly - I can understand your worries about giving it ti a 4-year-old.
I may be in this position in a few years, as it's hereditary, and, despite my own appreciation of Ritalin, would only give it to DS if his behaviour/attention/achievement was REALLY poor.
There will most likely be few consequences of giving him the meds, as they're very safe (Ritalin's been used for over 50 years with, AFAIK, no deaths from proper usage (ie legal).
It's not addictive, as the doses peak after 2 hours and wear off - it's designed that way.
If you do decide to go down the med avenue, don't choose sustained release, as this option doesn't have the built-in addiction...errr...proofness (sorry!).
Good luck, and let us know what happens.