Droid 4's touch screen seems to become a little unstable while the phone is plugged to the charger. The screen becomes a little insensitive to touch or lags.
It's a known issue, and it's worse with non motorola Chargers than with stock Motorola charger. The problem is worse with some owners' phones than it is with others' and some have virtually no issues regarding it, but it is common among the Motorola Android phones.
I have a charger in every room of my house. I can use the phone just fine with all of them. Some aren't even motor chargers. But I have one moto house charger that I keep in my truck. If I plug that house charger into the inverter that plugs into my cigarette lighter and plug it into my phone, I can't use my phone at all.