Thanks. Look in your mobo manual p. 27 for the configuration options available for EACH of the two SYS_FAN headers. Each of them needs to be adjusted. You have four fans, you say, and 2 of the 4 somehow change speed but not the other two. I am assuming that these four ALL are your case ventilation fans, and we are not talking about the cooling devices on your CPU chip. Is that right? THEN, since you have 4 such fans, how are they connected to two headers? Are you using simple Splitters to attach two to each of the SYS_FAN headers?
I will assume Yes for both those questions in this post. IF I am wrong, post back here the correct info for adjustment of my suggestions.
For EACH of the two SYS_FAN headers, select that header and configure as follows.
Fan Speed Control Normal
Fan Control Use Temperature Input to Motherboard,not CPU
Fan Control Mode to Voltage
Fan Stop Disabled
Temperature Warning Control 60C should be OK for the mobo, but if that generates warnings often maybe increase
Fan Fail Warning Enabled
When you have them both adjusted, use Esc to get to Main Menu, then F10 to get to Exit Menu (p. 38). Choose Save and Exit Setup to save and reboot.