I was going to say deworm her as well, because you comment on a huge belly at 4-5 months. I would probably do a fecal test first, especially since she's pregnant, and then see if the vet recommends Ivermectin or maybe a Power Pac to get her back to normal (if it's called for at all).
I've fed up skinny mares, pregnant and not, and I SWEAR by either Purina Senior or Purina Omolene 400 and all the hay they can hold. Alfalfa is great if you're in an area where it's safe to feed it, or even just unlimited grass hay. The trick that's worked best for me is to slowly increase their feed to where I WANT them to be rather than feeding for where they're AT.
So, using Omolene 400 as an example:
Let's say your mare weighs 800 lbs now and you want her to weigh 1200 lbs when she's up to weight. Omolene 400 is a complete feed so technically you don't need to feed hay with it, but when putting on weight, I feed the 400 PLUS all the grass hay they will eat.
So, too keep her at 800 lbs, you'd feed 8.75 lbs of 400 per day but to get her to 1200 lbs, you need to work up to feeding her 13.25 lbs per day. So if you feed twice a day, you'd add 1 lb per feeding, so go from 4.5 lbs to 5.5 lbs per feeding for about 5 days, then up to 6.5 lbs for 5 more days, then go to 6.5 lbs and 6.75 lbs as your final feed amounts.
Once I get them to the weight I want them at, I cut back on the 400 a little, so they hold their weight. I gave you the weights to feed for a mare in early gestation, the first 250 days. Once she gets to late gestation, the last 90 days, the feed rates go up so you'd have to look at the feed tag and figure out a new progression.
Once they get to the last 90 days, I start switching over to Omolene 300 which is designed for lactating mares and foals. I keep the foals on it til they're about 6 months and then I switch to Purina Equine Junior til they're 2 years old.