GTA IV has a very tragic story and one of my favourite protagonists. GTA V is more upbeat. It has its tragic moment but I think the overall them is more excatic with big heists and the dynamic relationship between protagonists.
I personally would say IV, but if you're not up for a lot of feels, do GTA V.
Both are great games. I'd play GTA 5 if your PC is good enough, but I can't play it well anymore since all the DLC updates. GTA 4 is still great and I think the shooting system on that is better than GTA 5's.