Mid Republican Roman army list

I am trying to get to grips with this army list (6.6)
It has 2-4 Legions which can have 2-4 Hastati and Principes VBU6/4 VD3/1.
Is this 2-4 units of Hastati AND 2-4 units of Principes or 2-4 LARGE units of HAstati and Principi combined?

I read that the Hastai and Principe can exchange places, but how does this work with a large unit? If casualties are taken from the rear unit why would you swap places?

Any help in clarifying this would be welcome!
Thanks
Philip

They are allowed to form large units as a way to simulate the front troops relief.

I understand that, but if you do NOT wish to form a large unit what are the stats for the two types, Hastati and Principe? And how many of each can you have in a legion 4?
Thanks
Philip

Maximum per legion is 4 "single" units. So, if you choose to have all hastati/princeps formed into large units then you can have a maximum of 2 LU's per legion.

That's great. Many thanks.