Choose from one of the 2 options

Use OpenIV or SparkIV to place the contents of your option here:
GTAIV\pc\models\cdimages\weapons.img\

If you want to replace the pump-action short-barrelled shotgun, rename the spas-12 files to w_shotgun before placing them in weapons.img

MAKE SURE YOU BACKUP YOUR VANILLA FILES AS YOU CAN'T UNDO THE MOD ONCE YOU'VE REPLACED IT