What do you mean by "does not blow like one?". Be specific...really specific.
If you are just talking about limiting the current to 4A rather than disconnecting it, keep in mind that voltage will have to be reduced to accomplish this. If this is not acceptable, then your task is impossible. If it is, what you do is get an your microcontroller (or whatever smarts is controlling the PWM on an H-bridge) and equip it with a current sensor (ie. hall effect current sensor, or a current shunt resistor with a current sense amplifier IC if needed), and program it to reduce the PWM duty cycle to keep the current at a certain level if it exceeds this level.
So, no, you can't buy one. Make a series-connected device that does this (like a TVS diode, but for current) and you'll be rich.