Tonya, if she's only been in training for 4 months, she's still building up muscle, so will be low on staying power, but that's just normal baby stuff. If you really think she could use a boost, you could try Red Cell, but, there's various schools of thought on that one. Most horses aren't iron deficient, and they are natural blood dopers. I supplement Lysine, which is an essential amino acid which is the one most likely to be deficient horses and humans.
But, if it's just that she's a baby building muscle... the only cure is time!