I suspect you're on the wrong track. Instead of messing with the truck's suspension, you need a weight-distributing (WD) hitch. Assuming your boat trailer has surge brakes, then you don't have a lot of choices as to which WD hitch to get. Equal-I-zer is the only one I know that advertises they work with surge brakes.
With a gross wet and loaded trailer weight of less than 6,000 pounds, you need the Equal-I-Zer hitch for 6,000 pounds max trailer weight (600 pounds max hitch weight). That's the lighest-duty WD hitch Equal-I-Zer makes, and it's just perfect for your requirements. Here is one source:
If your boat trailer has a pole design instead of an a-frame design for the hitch and coupler, then you may need a slightly different hitch than the one I linked to. But they're available, so do your homework to get the correct hitch for your trailer.
No, normally a 4,000-pound boat trailer does not require a WD hitch. But a WD hitch can fix the complaints you have, so why not install one?