These things have helped me improove to the point where i can literally see the improvements.
1)Practice everyday
2)Start with a good warm up that covers, stretches, scales, hammer ons and pull offs, tremelo picking, and if you want some harmonics.
3)Dont learn licks or riffs, learn the whole song.
4)If you find a hard riff or chorus, break it down. Music class thought me this ****. cut it up into small parts and learn them individually it really honestly works.
Thats about it. I can honestly say that by doing this everyday, and trying to learn a new song every few days I've really started to improove alot in the past 4 weeks.
Now since you've been playing for 4 years...you should do much better than I did.