Here is a list of all the unlockable weapons in Call of Duty 4 there is also the rank to let you know when you get the weapon.
You can unlock the following weapons in online multiplayer mode, when you have reached the numbered rank listed.

• Assault Rifle (G3) - Rank 25
• Assault Rifle (G36C) - Rank 37
• Assault Rifle (M14) - Rank 46
• Assault Rifle (M4) - Rank 10
• Assault Rifle (MP44) - Rank 52
• LMG (M60E4) - Rank 19
• Pistol (Desert Eagle) - Rank 43
• Pistol(Golden Desert Eagle) - Rank 55
• Pistol(M1911) - Rank 16
• Shotgun(M1014) - Rank 31
• SMG(AK-74U) - Rank 28
• SMG(Mini Uzi) - Rank 13
• SMG(P90) - Rank 40
• Sniper Rifle (Barret) - Rank 49
• Sniper Rifle (Dragunov) - Rank 22
• Sniper Rifle (M40) - Rank 04
• Sniper Rifle (R700) - Rank 34

Here is a list of the following “Golden” weapons you can unlock in the online multiplayer mode, when you have reached the numbered rank listed or if you have completed the challenges.

• AK-47 - Complete all Assault Rifle challenges
• Desert Eagle - Attain Level 55
• Dragonuv - Complete all Sniper Rifle challenges
• M1014 - Complete all Shotgun challenges
• M60 - Complete all LMG challenges
• Mini-Uzi - Complete all SMG challenges