SIM800L power interface
You could make a stable 4V from you 5V supply with voltage regulator, like TI's lm2596s.
It has an adjustable output version which is capable of driving a max 3A load with excellent line and load regulation, beside providing the adjusted fix voltage (like 4V in your case). 

I have used this IC with my GSM module which has similar supply voltage ranges like yours (3.4V - 4.6V, 4 recommended), it works fine.

Though, it requires more components and more space than a single zener diode, I think it is more reliable.



The datasheet has a good Design Component Selection Guide. Also, you can request samples if you have an educational or company e-mail address.