Sudden jerking movement bet. 20 mph to 40 mph.

What is your question?

Could this be a problem with the automatic (D) transmission when shifting from 2nd to 4th gear? It normally happens when the RPM is around 2,000. If it is a higher RPM often it does not happen. Once I get past 40 mph and above, it does not happen anymore. If I am at 40 mph and would slow down to about 20 mph, after I step on the gas again, it feels like it is at 4th gear when it should be in 3rd. Please help. Thanks!