A few months ago I uploaded a CS:GO weapons pack, which included the default versions of the M4, AK-47, Desert Eagle, etc. I included the M4A1-S too. The mod you just downloaded includes the unsuppressed version of the M4A1-S, along with some of the skins Global offensive features. You can use the skins included in both the M4A1 and M4A1-S I uploaded in the CS:GO pack. I'm expecting to upload each weapon separately to fit with this mod.

Installation:
1. In OpenIV, open weapons.img located in the following location:
GTA IV / pc /models / cdimages /
2. Enable Edit mode and then click on "add new file".
3. Choose both .wdr and .wtd files located in the root of the mod folder.

(Optional)
*Adding a skin*
4. While running OpenIV, right click in w_m4.wtd and click replace.
5. Search for the Alt Skins folder, open it and choose the skin you prefer. (I couldn't add a pic so you can preview the skin, but I hope you already know the skin names u.u).

Thanks a lot for downloading this mod, you support me really much this way.

Credits:
Credit goes to Valve and Hidden Entertainment for creating the M4A1-S model and default texture.
For the included skins, the credit goes to all those talented artists who developed these beautiful skins.

With that mentioned, I take no credit for creating any model or skin included. I only take credit for porting these weapons into GTA IV.