These tanks are found in the American Tree. Ignoring the first 3 tanks as they only cost a little bit xp and silver and won't cause any issues, it gets a bit more to advance to the next tank once you get to the 4th tank (M3 Lee) in the tree so I have detailed the xp and silver requirements below.
The M4 requires 14055xp using the M3 Lee or free xp and costs 350500 silver.
The M4A3E8 will require 30500xp using the M4 or free xp and costs 930500 silver.
The M4A3E2 requires 32750xp using the M4 or free xp and costs 975000 silver.
So its not exactly an easy achievement and will take a bit of grinding with the M4 to get the 63250xp and 1,905,500 silver for the last two tanks.