U-turn one day and it started misfiring

Hi I have a 1999 Hyundai Elantra. I baught it 3 years ago from a family after the owner had passed away he supposedly was sick and had this sitting in a parking grage for a while so it had 63,000km on it so of course i jumped on it for only 2000$. It ran great until i did a U-turn one day and it started misfiring so i brought it to a random dealership and they replaced all my spark plugs with no result, one guy then leaned over and said he heard a sound and said i had to replace my engine. of course at that time i was like wtf ill go see someone else first. so i went to my small home town mechanic that was a little more trust worthy and he said it was probably sediment from the gas tank because it was sitting in the parking garage for what could have been years. he then cleaned my injectors and it worked perfectly. about a year later i got in an accident just a little fender bender just had to replace my drivers side headlight and my insurance went up so i decided to pull my car off the road for a bit. 1 year later i decided i wanted to drive again so i go and do a tine up and change my fluids and put some fuel line cleaner through the engine. and of course it stared misfiring (cylinder 3). I brought it to my mechanic and he did the same thing once again cleaned my injectors and it ran good..... for 2 days from then on it was misfiring constantly i brought it back to him and he basically gave up and tried to sell me a new car. so then last night i started up my car and started driving when it started shaking like crazy till a point it stalled out. i pulled over and stared it up again but it just shook and stalled out. i scanned the code after this to see what is wrong and now it is misfiring in cylinder 4 so of course the car won't run on 2 cylinders. can anyone help me to figure what is wrong ?

Great site :)