Garlic is a bulb that spreads by generating bulbets (cloves).

What would happen if you planted a clove of garlic and then just left it there for 3-4 years?

It would slowly spread outwards over time with a mass of plants all clustered together. Just like daffodils etc.

It will grow based upon the environmental conditions. It's literally one of the easiest crops to grow because of it. You can plant it in July and if it's hot it will remain dormant for a month until cooler temps.

Yeah I probably wasn't clear enough there - it grows perfectly fine in British weather, so don't water it unless there's a drought, the rain will do the job fine. Unless you have some kind of particularly bad soil, it should be as simple as planting bulbs and leaving them there until harvest.

I asked my dad about mulching and he said "I dunno, I always just stuck 'em in the ground", so I'm gonna say probably not necessary - when he grew it on the allotment we never lacked for garlic.