You can't upgrade the graphics hardware of an MBP and that settles that.
MacBook Pro's have great resale value. The only MBPro with a compatible card is the new Retina model. A trade-in is your only viable option.
Ok so say I were to sell it what would you recommend I buy for best performance with a budget of no more than 4K.
If you need portability and want to stick with a Mac then the Retina display is the way to go. I'm very happy with mine. If you don't need portability and want to stay Mac then a Mac Pro would be the way to go. More cores and more performance for the $4K.
If you want to move to Windows then you've got to be just as careful about compatibility and video cards with laptops. I'd also suggest that you go with a major manufacturer if you go for a desktop. These days you can get a pretty amazing machine for $4K....