This is very common; but it is key to keep going. Are you keeping track of inches and sizes? If you are still losing in that way, you are not in a full plateau. Sometimes, when the scale isn't moving -- I was still losing inches and sizes. Those are fab NSV (non-scale victories) that were/are very encouraging to me. I have kept up my exercise & toning plan, and I'm sure that is what helped that.
Now, for the scale -- maybe shake things up a bit; cycle your calories or your food plan a bit. Not sure what kind of plan you are on, but it's time for a "tweak" session for sure. Maybe eat more veggies/salads, fruit, and lean proteins; and maybe do some substitutions.
If you haven't increased your food intake, that 4 lbs could very likely be water gain; that happens a lot too. In time, that will go back down.
The most important thing is to keep going, the scale will budge eventually ...
