It doesn't matter. Old Lua scripts will still work in 5.x, but you now have some newer methods available too for extra shorthand.
While 5.x might look a lot different to 4.2.5 & below, in large part due to the new GUI, it still more or less works the same. I recommend starting with 5.x for many reasons. 4.2.5 is outdated, has various unaddressed bugs & will not be getting any new updates to fix said bugs.
Now in regards to Lua script... do you already know any scripting/programming languages or is this the first scripting language you are learning? if it is then I'd just focus on learning how to use Visionaire Studio itself first rather than diving ass first into the deep end (as I like to call it).