Sounds like a plan.  You'll probably getting bigger gains now and lower gains later, but your average will be good.
Depending on how big you want them and how soon in the spring, I'd still hesitate to go over 4# of the mix.  When the grass does run out you can feed more according to their flesh condition along with the hay.  Save your better hay for the cows, these calves won't need anything fancy for hay with the mix you have to feed.