4.09 is important because it was the last version of IL-2 1946 before Team Daidalos took over the "official" development of further patches. Not everyone is happy with what TD did, and those who aren't are most happy with the latest version before that incident.
At the same time, 4.09 was the first version to see a very successful mod package called "Ultrapack 2" to be applied to.
4.10 is important because it saw the first really "all inclusive" mod packages for both off- and online gameplay (DBW and Ultrapack 3).
It's still important because we kept the Ultrapack development going and our SAS Gameserver for instance is running UP3 + Patch Pack.
There's a set of features available in UP3 which are quite genuine, most notably the full featured "Zuti Moving Dogfight Server" (MDS).
4.11 is utterly unimportant.
4.12 is important because it's the base for the most comprehensive modpack ever seen - BAT.
Furthermore it's also the base for another modpack - VP - which is focused on WW2 content.
4.13 is currently the last official patch version, but it's been mod-unfriendly to an extent that we decided not to follow that route anymore.
We do feature a mod activator ("Modact") for it and there are mods available for this version (some here, some elsewhere), but you won't find anything that remotely compares to what is available in BAT and VP modpack for instance.
4.14... let's see what we get when it's done and if it's ever done.
Mike