> How do you see thats its limited to only 4A?
It stated in its datasheet. LTSpice do not take in account physical limits of devices which it [LTSpice] simulate.
> Could you please reccomend me a PMOS model i could use?
You should firstly select REAL transistor, by its real parameters. Ids and Rdson firstly.
What maximum voltage drop on MOSFET you can tolerate? 0.1V (0.5% of Supply Voltage for AMP) will be acceptable?
And what budget limit for MOSFET? High power MOSFETs can be expensive
SISS65DN-T1-GE3 (0.8 euro on Digikey). Gate maximum voltage is 20V , so additional zener should be used to protect Gate
FDS6681Z (1.49 euro). Gate maximum 25V, no zener needed