SMPS Constant Current Source only works with low resistance loads
Your feedback op-amp U1 is only going to work correctly if its input voltages are between its supply rails. That is, 0 to 12 V.

If you try to push 4.5 A into a 4 ohm load, you'll have the output voltage at 18 V , so the U1 inputs will be at about \$\frac{21}{22}(18\ V)\$ and U1 will stop working correctly.

Try using a higher voltage to power U1 (which might require choosing an op-amp that allows higher voltage power supplies if you want to build this circuit in the real world).