You misunderstand the signals from a PWM header. In that signal system, the power supply is Ground on Pin #1, and +12 VDC constant VOLTAGE on P{in #2. The PWM signal is sent on Pin #4. Inside the fan a chip uses that signal to modify the flow of CURRENT from the 12 VDC supply line through the windings to achieve speed control. So IF the PWM signal calls for max speed, each fan will then draw its max AMPS rating of current from that 12 VDC supply. When you use a SPLITTER to connect several fans to one header, the AMPS current drawn by each fan (IF they are the SAME fan model) is the same, so total AMPS is max for one times number of fans.
In your case with FOUR fans plugged into the DeepCool Splitter there MIGHT be a problem, but likely not. Why? Well, the specs for those fans say max current is 0.30 A per fan. 4 x 0.30 = 1.20 A, and that may exceed the Splitter's ability. BUT there is a big unknown here. The spec does NOT say that amp rating is for the MOTOR only! It is a LIGHTED fan with ARGB lights in its frame, and the lights themselves draw current BUT from a different supply through a separate cable from a different mobo header. The specs do not say anything about the max current for the lights only. So the max current spec published MAY be the total for BOTH motor and lights, and the motor is less than that. Those specs simply to NOT tell us this detail. It is common that fan MOTORS of this size and type draw a max current of 0.10 to 0.25 A per motor, LESS that the 0.30 A in the published spec. So, what IS the REAL motor max current draw? We do not know.