The pistol has right sound so it should've been actually already useable in GTA IV but it doesn't have model and weaponinfo.xml 
lines at the start but has only sound so i have brought it to you for use, no need to use your trainer.ini to get that weapon sound the game has it already,
this sound includes with the model w_silenced. but remember to enable episodic weapons from your trainer.ini before starting the game.


add these lines into your weaponinfo.xml in <GTA IV/common/data> folder

	<weapon type="EPISODIC_4">
		<data slot="handgun" firetype="instant_hit" damagetype="bullet" group="pistol_small" targetrange="45.0" weaponrange="50.0" clipsize="17" ammomax="1500" timebetweenshots="333">
			<damage base="25" networkplayermod="3.0" networkpedmod="1.0"/>
			<physics force="50.0"/>
			<reload time="2000" fasttime="2000" crouchtime="2000"/>

			<aiming accuracy="0.5">
				<offset         x="0.15" y="1.0" z="0.55"/>
				<crouchedoffset x="0.2"  y="1.0" z="0.05"/>
				<reticule standing="0.65" ducked="0.55" scale="0.3"/>
			</aiming>

			<pickup regentime="120000" ammoonstreet="30"/>

			<controller>
				<rumble duration="110" intensity="0.02"/>
			</controller>

			<flags>
				<flag>gun</flag>
				<flag>can_aim</flag>
				<flag>can_free_aim</flag>
				<flag>anim_reload</flag>
				<flag>anim_crouch_fire</flag>
				<flag>SILENCED</flag>
			</flags>
		</data>

		<assets model="w_silenced">
			<anim group="gun@handgun" meleegroup1="firearm_core"/>

			<effects>
				<muzzle fx="muz_pistol_small"/>
				<shell fx="weap_ejected_pistol"/>
			</effects>
		</assets>
	</weapon>

add these lines into your default.ide in <GTA IV/common/data> folder
{
Weap
###############################################################################################
w_silenced, 		w_silenced, 		gun@handgun, 		1, 	30, 	0


###############################################################################################
amat
###############################################################################################
w_silenced,		0,		CM_WEAPONS_PISTOL
}

add w_silenced.wdr and w_silenced.wtd to <GTAIV/pc/models/cdimages/weapons.img>

Make sure to start a new game to avoid any unecessary bugs.

download & Install simple native trainer if you haven't done so already
https://www.gtainside.com/en/gta4/trainers/29661-simple-native-trainer-v6-0/

search in trainer.ini episodic=0 then set it 1 save and close
go ingame open trainer by pressing f3 then navigate to weapons -> Select Episodic Weapon Episodic 4

Thanks for Downloading