The current version is 5.1.x which has been named LTS. Before 5.x was released, 4.2 was the version a lot of people used. It doesn't matter exactly which version they used (although 5.1.x is a better answer to hear), but it's nice to hear how they talk about the different versions.

If they say they used to use 4.x but now use 5.x here are some potential questions:

Do you like the new folder structure introduced in 5.0?

Did you migrate any existing 4.x applications to 5.x? If yes, tell me about that.