I have a Sandpiper and love mine. They are very good quality trailers. They might be slightly heavier, but that's not necessarily a bad thing. The lighter they are sometimes means cheaper quality construction. Your truck will pull 4,000 pounds about the same as it will pull 5,000 lbs. Just make sure you keep your loaded weight under what your truck is rated for. Usually 1/2 tons are rated to pull between 7,000 and 9,000 lbs.
WW makes a nice trailer too, but I have heard quite a few complaints from owners on them. On the other hand I hear they have good customer svc, but these people might not know this unless they had a problem with their trailers to begin with.
I'd say they are both pretty comparable trailers, you should be happy with either one.
Those prices seem way high for a 18 footer though! Do your homework and shop around before you buy one. You can usually get an almost new toyhauler (used) for THOUSANDS less.