Selected category updates sub category dropdown menu

Been searching for a couple of days now and thought it's best to ask the experts as it's driving me bonkers.

I'm looking to build a parts catalogue (with no online ordering just yet but will possibly add it in the future) and don't seem to be able to work it out.

Basically I want to have a drop down menu to update and change to the next sub category when a parent category is chosen.

Not quite the perfect example as this uses 4 drop down menu's whereas I'd like to just use the one and it auto populate but this gives you an understanding roughly what I'm trying to achieve.

Really hope somebody has a simple solution that I've missed.

Thanks all.