I can't believe no one has said this. You liked your old "pre-modification" recipe, right? But you just want less of it? Your eggs are the same size as always?
3egg whites/4egg whites=X/500g icing sugar. Remember learning cross multiplication in school?
Assuming your eggs are the same size: if four egg whites marry well with 500 grams of sugar, then three egg whites will behave the same way with 375 grams of sugar.
To have a sense of accuracy:
Weigh 2 egg whites together - note that weight and then note that same weight divided by two.
Add a third egg white, weigh the total. Take the total weight of three egg whites, divide by three. That's an unnecessary step, but it will give you an even better idea of margin of error.
So the weight in grams of two egg whites divided by two should be about equal to the weight of three egg whites divided by three. Get it? If they're significantly different, take the average of three and run with that.
So now you know not only the total weight of your egg whites, but also the average weight of your egg whites. Knowing that average weight can allow accuracy regardless of the weight of future whites. Just replace #ofeggwhites with total-weight-of-egg-whites, so grams instead of #ofeggwhites.
So if you don't want to weigh your egg whites, you can at least estimate. (That is of course eliminating such things as weather from the equation, but it's a great place to start.)