Currently most TFW services can cope as a 3 car. At peak times you may see people standing, but not at the way a 2 car means you're either left behind or in someone's armpit. When TFW win travellers back with a more consistent service with enough capacity then 4 or 5 car will probably be more required. Don't forget that Mk4's now are only 2.5 carriages of standard and 1 carriage of First, so unless First is declassified, a Mk4 is not a 4 carriage train.
A 4 car Mk4 has 182 Standard and 40 First seats (222). A 5 car Mk will have 258 Standard and 40 First (298)
By comparison a 2 car 197 has 118, 3 car 197 186 and 3 car with First (Business) 158. So a 5 car 197 with First will by comparison have 276 seats.