The impedance seen at the base is a diode in series with R3*Hfe, which will be very high, compared to R1, so can be ignored.

The transistor will just turn on as much as it can, giving a saturation voltage of a couple of hundred mV. The voltages across R3 and R4 can be calculated using Ohm's law.

I = (V1-V_{SAT})/R3+R4)

V = I*R

Where V_{SAT} is the saturation voltage of the transistor, which will be around 200mV. If you're unsure, presume it to be zero, calculate I, then look up the value on the data sheet and recalculate. This will give as gooder result as you're going to get, as the saturation voltage isn't 100% predictable and can vary from device to device.