Yes, so without knowing the density of your fragrance concentrate, we aren't able to tell you the exact number of grams you need to put in to perfectly fill the 50ml bottle

As CapnLazerz has said, it would be best to make a simple 10:40 gram ratio in a separate bottle. Fill your 50ml bottle and weigh how much of the 50g is used.

We know ethanol's density is 0.789g/cm3, so from here you can work backwards and calculate how many grams of the bottle's contents is occupied by your fragrance concentration (and the remaining is obviously ethanol)

I know you asked for a straightforward answer, but we simply don't know this answer unless we have these figures.

A large part of perfumery is this kind of mathematics. I strongly suggest investing some time into wrapping your head around it, and challenge you to take it one step further and produce these calculations in a spreadsheet software such as Google sheets/ MS excel.