Just make a proportion. Four tenths is what fraction of a milliliter? Four tenths equals how many microliters over one thousand microliters?
(Ignore the dots please)
4 ....... x μL
— =....—
10….1000μL
Cross Multiply so that 4000μL=10xμL. Now just make it so that x is on one side of the equation and you’ll get the amount of the protein in the dilution.

I think you are misinterpreting what is meant by dilution factor. By your reasoning, a dilution factor of 325:100 would give a resulting concentration of over 3 times the original. That can’t be right. It should be that, for example,1:2 means 1 volume of protein to 2 volumes of buffer. That would give ⅓ ml of protein and ⅔ ml of buffer. The other protein concentrations would be 4/14 ml, 325/425 ml and 175/275 ml.

@LostInParadise Well, technically this a serial dilution. I should have made that clear in the question. I see your problem as I’m trying to do what @dxs told me. Although, what you’re saying about 1:2 confused me – my boss directly told me what to do, and he told me that it was .5ml of protein and .5ml of buffer for the 1:2 dilution. He gave me a hint as to the 4:10, something about it being 80% of the 1:2 dilution . . .

Unless. . . . oh crap, I’m sorry. 1:2 is a ratio. I mean ½ dilution. So meaning half a dilution of the original stock solution.

This doesn’t make sense then. Each dilution tells me really how many milliliters I need. For example, ½ = .5 So it’s going to be .5ml and .5ml. ¼ is .25, so the other half will be .75. 1/10 is .1, so the rest will be .9. 325/100 would give me more than 1ml. Since it would be 3.25. As would 175.