A couple of things:
Are you playing in a local server which gives you a low ping?
Have you tried a hardcore server?
When firing any rifle in the game, I aim basically at the opponents belt line and fire in bursts of 2 to 3 shots. Recoil causes the barrel to jump substantially and thus the next several shots hit mid chest.
There are several that have little to no recoil and you can basically sit there holding the trigger down while aimed at mid chest and get kills though.
Good guns based on classes:
Assault:
AK-12: great gun. Very easy to rack up kills with.
SCAR-H: I have not used yet.
M416 also very well balanced.
SAR-21: current gun and liking it also.
Engineer:
MX4: seems decent all around, quick to get up to sights, not too much recoil
I have not used any other PDW yet.
Support:
Personally, I have stayed away from the LMGs, I could not for the life of me kill anyone with them.
Recon:
I wont even begin to touch on the sniper rifles, they are an entirely separate beast.
All classes can use the Carbines, once you unlock them. They are (IMO) by far the best rifles for overall use.
AK 5C: great starter carbine, loved it.
ACW-R: Awesome carbine! Still great just like in BF3.
SG553: it was great in BF3 and I have heard lots of positive feedback regarding the BF4 version, but I have not personally tried it.
ACE 52 QCB: slower firing carbine, but it seems to have some stopping power. I would say its a mid range gun based on usability and kills.
G36C: It seems like they have buffed this carbine since BF3. I am liking it AFTER you get done with its horrid iron sights.
Also regarding normal servers, I find them to be frustrating for several reasons:
The spotting of enemy troops is better than BF3 (ie when you go behind cover the marker doesn't linger quite as long) but its still absurd IMO.
It does seem like they have reduced the amount of hits needed to kill someone in BF4 vs BF3 for normal mode, but its still a lot. AS for the one shot scenario, it may seem like it, but they are experiencing the exact thing you are. Most likely they are unloading half their magazine into you as well. Next time you die, look at the health left of the enemy who killed you to see how close you came to killing them. You will be surprised..