Not a Dexter keeper, we have commercial sucklers (Angus & Hereford mainly, and a few Blue Greys) and house Jerseys.
Is this her first calf? Is it a Dexter bull? Is she sharing the 4 acres with anything else?
As Rosemary says, any pregnant or lactating cattle should have access to the green Crystalyx and to rock salt.
I am not familiar with your area and its climate - hereabouts, all cattle except the hardiest are needing extra forage by now. We're about to move our Blue Greys to better ground, or we would have to start haying even them.
If she's calving in January she should be starting to bag up about now. I'd keep an eye on her pin bones - if she's a layer of fat on them she's not wanting much, but if it's her first calf I'd maybe be giving her just a little cake (a pound or two, no more) to make sure there'll be enough milk for the young 'un. And, me being me, I'd have plenty of nice hay on hand to feed her ad lib once the calf is born.