Hi Lance.
A 4mm GSO Plossl on it's own will be most unsatisfactory 9 times out of 10.. barlowing it! omg, that's just asking for trouble!
What type of eyepiece are you looking for exactly? A planetary? or a deep space eyepiece to use barlowed as a planetary?
The 11mm widefield from Andrews is probably not the best choice in a fast dob for planets.. firstly, 11mm is too long for detailed views (not enough mag), and secondly the edge of the field of view will not show sharp views, so whenever the planet is near the edge it will distort badly.
For planetary, i'd be looking at something of reasonably quality in the 5mm-7mm range. These include UO HD Orthos ($125 each), Vixen LV's ($170ish each), or the new Burgess/TMB planetary eyepieces (probably around $150+ each). The Meade s5000 plossls would probably be ok too.
At least you only wasted $40, but I'd be careful about wasting more by buying the cheap widefields for use in a fast dob.