Not possible with stock LFS. However the cars in LFS are built from a modular architecture. Third party applications (tweaks) allow you to change certain parameters. So you can turn the RB4's engine into a 5-cylinder and the game will render the sound accordingly!
That's the same as people always wanted DX11 or DX12 or whatever, thinking it directly gives super shiny and sharp graphics.
ETS2 and ATS recently switched to FMOD for the sound. It does add some nice new effects their previous engine didn't support, but it's not like it's a night and day difference. They mostly use it for nicer ambient sounds with easy doppler support and echoes from tunnels and overpasses. But the trucks still sound the same.
Depends on the tweak you use, but most commonly there's paragraphs for minimum and maximum cylinder value in tweak that you can change around, and the default limit on most of the cars is 12, with exception of few single car classes and LRF.
Just increase that max amount and you can get 12+ cylinders no problem
As for the engine sounds, it is possible to change the engine pulse sample which can give a completely new tone to a car, but i'm unsure if it's possible to use it to make an inline 4 sound like a V6, for example. But the samples itself are a bit hard to work with, so far i've ended up with massive amounts of sound clipping after i've tried different samples
CSR is great for singleplayer and especially when driving onboard, as it lacks the effects that LFS' sound engine has nowadays, such as doppler. I also think when using it online, it basically recognises cars on track and play their audio as well, because it doesn't really get any positional info from the cars on track so all the sounds are playing on top of each other at same volume. Yamakawa also made a great mod for south city that added a lot of ambient sounds into the game making it sound a lot more lively, and it's a shame that doesn't work either anymore.