Dec, I think part of what you are experiencing is the difference between how unlocks work in BF3 vs. BFBC2.
Unlike BFBC2, the starting guns in BF3 are side-specific. The M4A1 is the starting gun for the US engineer. Anytime you play as Russian, you will not have access to that gun. This is by design and working as intended. Any subsequently unlocked weapons will be available to both sides, and later on you can actually unlock the M4A1 on the Russian side, too.
Also unlike BFBC2, the weapon upgrades are gun-specific. So if you unlock a new holo sight for your M4A1, it will ONLY be available for that weapon, and no others. You acquire upgrades for a particular gun by getting kills with that gun. You never lose those upgrades, but when you unlock a new weapon, you essentially start from scratch with it.
To see how you are progressing in weapon and upgrade unlocks, log into Battlelog, click stats at the top, then click Weapons & Equipment. You can then click on specific weapons on the right side and see what you've got coming for upgrades.