How to drop voltage to 4 volt from 5 volt source voltage with resistors?
The module requires 4.0V nominal (recommended) with up to 2A peak consumption and as low as a few mA when idle.
Forget about using a voltage divider. You would need to burn hundreds of watts in the divider to get reasonable regulation over that range.
You need a voltage regulator. A switchmode module based a chip such as the XL4016 would work, however the SMPS noise might interfere with the radio. The LM2596 modules are a bit marginal in terms of dropout voltage.
A linear LDO regulator capable of handling 2A peak with less than 1.2V dropout is possible, but they tend to be a bit expensive.
As well as the supply voltage you need to ensure inputs are kept within the safe range. You can use a voltage divider to reduce the TX output of the AVR to a safe voltage.