Rockstar really knows how to sell a game. Not only has GTA 5 recently shipped its 75 millionth copy to retailers (digital sales not included!), but the recent announcement that GTA 4 and Episodes from Liberty City are now available on the Xbox One via backwards compatibility jacked up the sales of those older titles as well.
Amazon, arguably the world's largest online retailer, is kind enough to list changes in the popularity of its products. The website indicates a marked increase in the sales of both vanilla GTA 4 and the complete edition.
Namely, 7,696% and 4,536%. Those numbers are not typos. These spikes prove not only that the backwards compatibility program on the Xbox One is a boon to publishers and players alike, giving old and seemingly outdated games a new spurt of life.

GTA 4 was first released in 2008 and is the first game in the "HD" era of GTA. That said, the game wasn't a pioneer of advanced graphical fidelity at the time like 5 was back in 2013. In fact, visuals were one of the most consistently criticized features of the game, being dark, and colors being muted.
GTA 4 instead won praise by virtue of polished, well-designed gameplay and what is lauded as the best plot and writing in a GTA game to date. Niko's tale is arguably the darkest in the franchise and hits a more somber and serious tone while still retaining that trademark GTA style humor.