A quick answer to this question would be: No. All the beaches within an hour or two drive from KL are somewhat dirty and unenticing. Port Dickson and Morib come to mind. The only form of public transportation to the beaches near KL is by bus. There are, however, plenty of world-class beaches within an hour's flight of KL (Langkawi, Pangkor, Penang, Tioman, Redang, Perhentian, etc.). If you want some beach time without too much hassle, Phuket is the better choice.

That said, KL will have more sightseeing opportunities and will most likely be cheaper than Phuket. Food and drinks in KL are excellent and cheap and transportation is transit-based. Ultimately, I think it all boils down to whether you want 4 days of beach time or 4 days of city time. You could do both (BKK/Phuket or KUL/Langkawi) but you'd be pretty rushed.