If you have played through the two stories of Episodes of Liberty City, one understands the whole story of GTA 4, which I personally found very good. There are also plenty of other activities you can do such as parachute jumps and many other challenges, where one is still employed neat ^^
Personally, I found The Lost and Damned on the difficulty more difficult than GTA 4. In comparison, the missions in The Ballad of Gay Tony were relatively easy. But overall I have to say that both of my stories have a lot of fun, because you also encounters old familiar characters, and sometimes very different sides of which looks.
In my opinion, this is again a masterpiece of Rockstar! The two stories are not as "serious" as the story of GTA 4, but each has the GTA 4 fallen Episodes from Liberty City itself should buy. And besides, I think it's good that you GTA 4 does not have to have to play this game .... is indeed not at all games so!
A 100% buy recommendation from me!
Have fun with it!