These are the installations you can download.
1)
IL-2 Sturmovik™: 1946 (Part 1 of 3) 4.13.4m 2 MB
IL-2 Sturmovik™: 1946 (Part 2 of 3) 4.13.4m 4 GB
IL-2 Sturmovik™: 1946 (Part 3 of 3) 4.13.4m 2.5 GB
2)
IL-2 Sturmovik 1946 4.10m Part 1 of 3 2 MB
IL-2 Sturmovik 1946 4.10m Part 2 of 3 3.9 GB
IL-2 Sturmovik 1946 4.10m Part 3 of 3 475 MB

What is the difference between them? Which one should i install.

If you are a new player, download and install 4.13.4m: it's the lastest one. Most mods will work with this one too.

The 4.10m is there for certain mods. Some ones, like B.A.T. needs from 4.12.2m, and there is a software which allow you to switch from 4.10m to any version.