The main differences between the 2 and 4 relay harness is safety and wig-wag function.
The 2 relay harness uses one relay for both low beams and one relay for both high beams so if the low beam relay or circuit breaker was to fail you would lose both low beams, same would hold true for the high beams.
The 4 relay harness has a relay and circuit breaker for each low beam and each high beam so if a relay or circuit breaker was to fail you would only lose that one light.
The 4 relay harness also allows the use of a wig-wag unit.
As a side note the factory headlight circuit uses one circuit breaker for both low and high beams so if it fails you will lose both low and both high beams.
You can use 80-100 watt bulbs with both harness.