From what I understand... the HD4800 series has plenty of horsepower for OpenCL applications... unfortunately it was produced before OpenCL was fully developed. Hence there's some math functions that are virtualized in the Graphics cards main memory pool, and not the local caches that are in the core.
Which is why an 800SP 5770 walks all over an 800SP 4850,70,90 in terms of hashrate, and not necessarily graphics performance.
And mind you this is in general terms, I looked and I can't quote the article I read it in. But I was interested why the HD3000 series couldn't run OpenCL apps at all (no local cache, if you were wondering.)