Ratios are for Iron: Copper: Zinc: Manganese --> 4: 1: 4: 4. With some wiggle room. Minimum amounts of Zinc are recommended at 400mg per day from NRC, making copper needing to be at last 100mg.
Generally, It is recommended to give little to no iron because unlike us, horses normally do not lose iron, unless they were to bleed or have some sort of pertaining condition that would exacerbate iron deficiency. In that way, anemia is rarer in horses than in humans. They also get iron from other sources, not just grass. Many supplements and feeds contain iron, those red mineral blocks and water often does as well, unless you have some sort of filter. Thus, these can add up and overload the copper and zinc.
Best way to really get the most accurate representation of what your horse is receiving is through both a hay analysis, pasture, and blood test. However, keep in mind that copper deficiency can often present itself as iron deficiency in blood tests. This is because Iron, in a way, depends on copper to be converted ferric state, which allows Iron to be more absorbable. An enzyme called hephaestin is what facilitates this, and it is dependent on copper. In other words, this copper-dependant enzyme permits release of iron from intestinal cells into the blood.
I think it is really good to check the ratios of all 4 minerals! Iron deficiency is possible, just be careful you don't overload the iron because that can also have unfavourable effects.