Exactly what model of subwoofer is involved?
The brief answer is "That won't work."
The subwoofer outputs on receivers are low-voltage line-level signals. They're designed to be plugged into external amplifiers which then drive the subwoofers. They won't work when connected directly to speakers. Powered subwoofers have amplifiers built into them (often called "plate amps") and are the type most often used. If the subs you have contain builtin amplifiers, the fact that they're 4 ohm speakers doesn't matter. If the subs you have do not have builtin amplifiers, either you have to purchase amplifers for them or they have to (somehow) be connected to the front speaker-level outputs of the receiver. The method for connecting them depends on the subwoofer design. Hence the need to know their exact models.