It depends on the car. All cars have different internal gearbox ratios, so the path you take to get to a 4.0 FDR will be different for everyone. For example, to get there with my Losi JRXS-R, the internal ratio is 1.83, so to have enough space for the motor to move close enough to the spur, I started with a 104t spur gear. A 47t pinion puts me right at 4.049 FDR.
I will probably swap out a 100t spur at some point, to allow a bit more range of pinions up and down, but for now, it works.
Some cars require really small spurs to get the proper ratios (many shaft drive cars like the TC3/TC4, Pro4, etc). Some cars with really low internal ratios (some Xrays are at 1.7, which is great for low turn spec motors) can get away with larger spurs and more modest sized pinions.
You can also change out pulleys in many cars to alter the internal ratios, too. Some cars like the Sakura Zeros and some Tamiya belt drive cars can really be dialed in for internal ratios, too, which can be a nice tuning aid for the really tall ratios we need for 21.5 and 25.5 motors.
Nothing is always.
Everything is sometimes.