The short answer is "maybe." Do a search - there's a lot of discussion on this issue here. The short version: the charging system doesn't kick in until 4k or so RPMs, and doesn't make good power until >5k. So if you're scared to twist the throttle, 4 amps probably isn't going to be enough. Same problem applies to getting stuck in traffic - if you're idling for an hour in stop/go traffic, 4 amps probably won't cover it (you can't really rev an engine at 5k on a non-moving air cooled bike for an hour and expect it not to seize).
That said, if you ride as you ought to (5-8k range or so), you don't ride at night and you don't get stuck in traffic - yeah, 4A ought to cover it.