From what I can recall, just the name. The 400A was just the Beechjet as we know it, Collins, glass cockpit and came about in 89ish from the original 400, which was known as "the straight 400" which was essentially the original product from Mitsubishi (the Diamond Jet) before it just had Beechcraft logos put on it and had steam gages and a slightly different fuel system.
The 400XP is just the 400A but after they started affixing "Hawker" to the jet line at Beechcraft, which was taken over by Hawker.
So an A is essentially the Beechjet, the XP is essentially a Beechjet masquerading as a Hawker. And the no designation was basically the original product (minus a few small details). Could be wrong, it's been a hot minute since I flew them, but going back to them as the current job isn't looking so hot insofar as allowing me to continue. Good airplane for what it's worth. 🙂