The HR02 has wider spacing than usual heatsinks, this makes it ideal cooler for low airflow applications, i honestly think you should be fine with the R4 and passive HR02 on idle, but load idk, i guess depends on what your server will be doing and what kind of airflow you will have on your R4. Personally i would suggest just to use the TY140 that comes with the HR02 macho, its a decent fan, i have been able to drop it to 700rpm on different mobos, so as long as your mobo can control pwm fans via the 4pin pwm fan connector (if your mobo has it) i think you should be fine, at max rpm 1300 the TY140 is loud.
Btw since you have R4 that has a white/black theme, in case you are interested, there is new version of the macho that only varies on the fan (from what i have read), comes with the TY147 that its the same specs at the TY140 but with white/black fan that should match well the R4 140mm fan color theme. Thermalright Macho Rev. A (BW) (=HR-02 Macho + Support Socket 2011 + Screw Driver)