That sounds like one of two possible problems:

1. You are not raising/lowering signals in the right sequence. I've seen a lot of inconsistency between what I've read in datasheets regarding when EN should be raised, lowered, and maintained in relation to when the data bus and/or RS are made valid or invalid.

2. You are not delaying long enough after the clear screen. While most data sent to the LCD can be processed in less than a few dozen microseconds, the clear screen command usually takes well over a millisecond.

If you have it working in 8-bit mode but not in 4-bit mode, I'd be most suspicious of #1. Of course, any recommendation is just speculation unless you post your source code.

Regards,
Craig Steiner