Can I upgrade from 4.3 to 5.0

Not sure what App is 4.3. The highest version of Safari 4 is 4.1.3, which is also the highest version of Safari that can run under OS 10.4.11.

Safari 5.0.6 is the highest level that can run on a PowerPC (basically pre-2006) Mac, and it require Mac OS 10.5.8 Leopard

The current level of Safari is 5.1.5 requiring Mac OS 10.6 or OS 10.7, both only running on Macs with Intel processors.