Grand Thieft Auto 4 was awaited by all gamers, not just GTA fans. I played only two missions, all other time was for modding, so can't say anything if it's good or bad by story. It's modern game, but not such fun as previous. Amount of bugs and very poor performance transformed many fans to haters. Unfortunately, about half of the year i could not play the game at all, because almost all objects where invisible, so active modding time was lost. First of all, vanilla game color filter and tonemapping are ugly, this was at first place in to fix list. No antialiasing is another bad side and problem of DirectX9 limitaions of deffered renderer used in the game, so also for modding. Textures (in general) in game are very nice, so high quality can be achieved by applying effects.
ENBSeries for GTA 4 is the most complex and long developed (because of game patches). I'm an idiot that spend so much time for it.
All versions have different look, so better to apply custom preset to proper version for which it's developed. Latest game patch is unsupported, i don't like reduced quality changes in it and don't have time now for adopting code. Best compability of ENBSeries with game pre 188.8.131.52 patches. But you may find on gtaforums.com project for adopting latest game version to ENBSeries.
You may see comparison screenshots from this mod and vanilla game in the gallery or on the gtaforums.com.