You should have received 4 year extension. Did you move since filing I-751? Maybe it got lost in the mail... To request one: https://egov.uscis.gov/e-request/displayNDNForm.do?sroPageType=ndn&entryPoint=init
You remain LPR, but you don't have proof of status. This is not a desirable situation.
I-751 used to take 3-6 months. But yes, it is normal nowadays to take 2-4 years.
Not sure what I-400 is, but if you're eligible, you can apply for N-400. Sometimes this pushes I-751.