Will a FMF Q4 make my FE 390 Quieter?

Quieter than what - the factory muffler? If that's your question, it's very doubtful. The factory cans are hugely restrictive. I imagine that the engine designers utterly despise the muffler guys. The Q4 will be quieter than the other two FMF mufflers available (I have the FMF 4.1), but I genuinely doubt that the Q4 will be quieter than the factory muffler.

I have the Q4 on my 2010 FE 450 and it is a few DB louder than stock. My Stock would run through a sound test at about 88 DB and with the Q4 on it goes through at 94 with the screen insert on the end of the can out and in the low 90's with the screen installed.

If your 390 is as loud as you indicate, your factory muffler is probably in need of a repack. But I wouldn't bother, as the factory cans are famously restrictive. I made a series of incremental internal modifications to the factory muffler on my 570, and eventually gave up on the thing. It's just a bad design.

I replaced it with an FMF 4.1 and run it with the reduced noise tail piece and the spark arrester, but I think it would be too loud for you. Without the reduced noise tail piece, the bike is jail bait. Even with the reduced noise part and spark arrester it's pretty raspy. I like it, but then I always ride with ear plugs.

You might give the FMF Q4 muffler a look. It's the quietest unit they sell for our machines, and it probably isn't so open that you'll need the performance map flashed to your ECU to keep it from running too lean and popping on deceleration.