I'm not sure, but I believe the last driver to support OCL on the 4xxx series of GPU was in March of 2014 (not sure of year, could have been the year before or after, fairly sure of the month). Look for legacy drivers notes to know for sure.
Largest Primes to Date:
As Double Checker: SR5 109208*5^1816285+1 Dgts-1,269,534
As Initial Finder: SR5 243944*5^1258576-1 Dgts-879,713