You can determine if 4 watts carrier is appropriate drive for the DX 500 using a good watt meter. If the AM dead key output is around 80-100 watts average final power out, you should be good to go. Anything over that will be pushing that 4 pill amp too hard IMO. The variable (if the amp has one) just pads the input and gets set so that you don't over drive the amp.
Some amps are touchy, so the difference between a 3 watt DK and 6 watt DK input is the difference between a clean sounding 1:4 ratio properly modulated output or an over-saturated, over-driven "sure to let the magic smoke out" situation. Unfortunately, I hear the latter all too often and witness cause & effect in real time over the air.
It usually goes down like this: Wow this thang is really puttin' out. look at that meter swang, oh yeah, Oh Yeah. Aud-d-o, Aud-D-O Now that's what I'm talking about!....uh oh, what just happened, hello...hello. Something just happened. I smell something burning. Hello? oh snap.