How to check if every part inside a model is close enough to another?

Inside a model there are more models and every one of them has a part called "Main". Every part must be close enough to at least one other part.

for _, model in ipairs(mainModel:GetChildren()) do
	closeEnough = false
	checkFor = model:WaitForChild("Main")
	for _, model2 in ipairs(mainModel:GetChildren()) do
		checkWith = model2:WaitForChild("Main")
		if <checkFor is closer to checkWith than 4 studs> then
			closeEnough = true
		end
	end
	if not closeEnough then
		script:Destroy()
	end
end

Obviously, I also check if checkFor~=checkWith, but I didn't include that here.

What I want to know is, how to check the distance between two parts, and if there is a more efficient way to do what I'm trying to accoplish.