AM peaking around 20w after tune should be about right for the 959 with mosfets. SSB should be around 18-20w if memory serves. I've tuned them up to do more when messing around but I never ran them that way, I keep them under 20 watts for both.
As Robb said radio should easily do the 4w key, that shouldn't be an issue.
As Robb said radio should easily do the 4w key, that shouldn't be an issue.