Cape Mac is fine for 3-4 days. Plenty of time for beach, snorkling, kayaking, relaxing (or not). Don't think you necessarily need to spend much time in LLW or BLZ though.

Note that Chileka Airport (BLZ) is currently going through emergency runway repairs, and flights are often delayed or cancelled during rain/wind/fog as they are having to use the short runway (Props only, no jet service at the moment). I would book flights to / from LLW if you are on a timeline that can't handle a potentially delayed return flight.

EDIT: Should clarify, when are you thinking of travelling?