Slow usb charger for my smart phone?
Is it possible to purchase a usb wall charger that charges at that voltage? Or perhaps 3.9v? I'm not finding anything on google.


The USB charger, as you call it, is not charging the battery. It's supplying 5V to the phone. The phone contains a battery charger, which controls charging of the battery. The  charger, inside the phone, regulates the voltage according to the state of charge and desired charging current.

The USB spec says 5V, +0.25V, -0.55V. Your 3.9V would be wildly outside the spec. A good phone would decide that the power source is not reliable, and do nothing with it. A bad phone may be damaged by it.