I have attempted to recreate the Beta M4AI (Carbine Rifle) and Desert Eagle (Combat Pistol) which are seen on CNT, the in-game TV channel.
I am 99% sure you're game must be downgraded. (Rememeber to move your game to a different folder in-case the platform you have IV on, re-downloads the files)  



--Installation--

1. Use OpenIV (or SparkIV) and find "Grand Theft Auto IV\pc\models\cdimages" and open "weapons.img".

2. Drag and drop everything in "Import to weapons.img" and replace the files (w_m4.wdr & w_m4.wtd, w_eagle.wdr and w_eagle.wtd)
 
3. Find "Grand Theft Auto IV\pc\anim\" and Open cutsprops.img

4. Drag and drop the contents of Import to cutsprops.img (csm4.wdr)

3. Find "Grand Theft Auto IV\pc\data\maps\props\residential\" and Open details.img

4. Drag and drop the contents of Import to details.img (cj_prop_m4.wdr & cj_prop_eagle.wdr)

Following is if you have either: EFLC/DLC + IV in one (Downgraded CE edition), or standalone EFLC)

5. Find TLAD\pc\anim\ and open "cutsprops.img" drag and drop the contents of "Import to (TLaD) cutsprops.img" (cseagle.wdr)
EFLC: cutsprops.img is in EFLC\pc\anim\ (or EFLC\TLAD\pc\anim\)
The Location could also vary to DLC\TLAD\content\pc\anim\ depending on the version/downgrader you used.

6. Find TBoGT\pc\models\cdimages and open "weapons_e2.img"
EFLC: weapons(_e2).img is in EFLC\pc\models\cdimages (or EFLC\TBoGT\pc\models\cdimage)
The Location could also vary to DLC\TBoGT\content\pc\models\cdimages depending on the version/downgrader you used.

7. Drag and drop everything in "Import to weapons.img" and replace the files (w_e2_eagle.wdr and w_e2_eagle.wtd)


--Mod Features--
. Edited Vanilla Models
. Prop Models (Ones that appear in Gun Vans)
. Cutscene Models

--Changes--
M4:
- Grey Grip and Reciever
- Slightly lighter grip and barrel
- Chamber Door/Flap has been removed
- Reciever has less parts on the right side

Desert Eagle:
- Less Detail In the texture
- Edited Back Knobs/Safety lever.
- the rear sight is just a copy of the front sight

--Bugs--
. The right side of the M4 is slightly broken and may have some tiny gaps.


--Other Notes--
I think i am mostly fine with this mod other than the right side of the M4 which was very poorly done and not entierley accurate. 
I may or may not revisit this mod in the future. I have more plans for non-beta mod related IV Mods and possibly might release one or two
mods for San Andreas and V. I have only uploaded this mod to gtainside, any other upload on any other site was not done by me without permission.
You can use assets from this mod as long as it's not a full repackage and I am credited as Golden Saddle. 
If you do want to include this with a bundle along with other mods, then let me know on twitter @GoldenSaddleYT or through the GTAforums and credit me.
Also check out the video I made showcasing this mod, pretty funny: https://www.youtube.com/watch?v=kyOT_GxOR6M
And if you want to  know who i got that "beta lighting feel" from the video, I used Zmenu and applied the Lobby2/Lobbyoffice or something timecyc modifer



