Hi,
No, use the arduino 5Vpin supply.
It will register as a HIGH, that is what the 5V pin is for.
Also the 5V will be noise free.
The transistor circuit will then be doing the level switching as well.
4.2V is still a HIGH.
I suspect you are over thinking everything. 
Read the Atmel spec sheets, they will tell you the threshold level for LOW to HIGH and HIGH to LOW transitions at the input.
Tom....