I just got my ex 1998 i4 5speed and does the same...
After 4500rpm it drops normally..
Lower than 4000 it keeps the rpm from the time you left the gas petal..
Under 1800 it works normally again...
it is definitely really annoying, i just got a 2002 i4 and its doing it the same as everyone elses. maybe i should just rev it to over 4000 and think its fine. lmfao i guess it make due for the winter until i get my other car back out