To qualify, I don't keg myself, so I'm only going on what I've read and what friends of mine do --- Yes you should be able to finish it out in a keg (short answer). Now for the questions - How cold is cold for the first 1.5 wks in primary, and what yeast did you pitch? What was the temp for remaining 2.5 wks? How confident are you in your original mash temps (assuming AG)? If you overshot your temps, there is always the possibility that your 1.021 is in fact your FG.
If the first 1.5 wks were well below ideal for the yeast, and the remaining 2.5 wks were on the lower end of temp, you may only need to warm it up a bit. Keep in mind that you will be working with far less active yeast since you racked it, so even if it gets re activated it might take what seems like forever to finish. You should be able to manually release pressure (if it builds pressure at all your probably looking good for finishing at a lower SG).