True, but note use a 5V zener (5.1V is usually the nearest stock value) to leave 7V at the fan as required, not the 7V one you've shown, and you've drawn it the wrong way round - banded (pointed) end of a zener goes to V+ supply, or it just drops 0.7V or so. The 5W zeners are a bit scarcer than 1.3W but if you can find a handy supplier, go for it.
You would need a much bigger diode there to make it work this way. 4x 120mm fans require more then 5 Watts. So you're looking for something like NTE5176A / NTE5177A (10 Watts).
See post #9 They're 3W fans @12V = 250mA, drawing approx 146mA @7V, four fans so 583mA flowing through zener, dropping 5.1V = 2.9W; plenty of safety factor left.
Ya, I had thought of using a 5w diode but I didn't want to be ordering from so many different sites as in the end it turns out to be more money from shipping so many orders.
I do realize the diode is backwards, I just used a quick online circuit tester and didn't know how to rotate it. Oops about the wrong rating. There are actually quite a few places I found selling 5W 5.1V zener diodes.
