Electro-Voice RE20 on a Scarlett Solo 4th Gen: is a booster needed?
Yes. At speaking level the RE20 needs about 57 dB, and the Scarlett Solo 4th Gen publishes 57, so 0 dB of gain is spare. Those are the published figures.
The Electro-Voice RE20 has enough preamp on the Focusrite Scarlett Solo 4th Gen for a normal speaking voice. You do not have to add a box in front of it.
Electro-Voice rates the RE20 at -56.5 dBV/Pa, the voltage from a loud 94 dB tone at the grille. A speaking voice is closer to 85 dB, so the preamp still has to find about 57 dB to land near −9 dBV. That −9 dBV figure is just a tracking level: high enough to use, with space before the converter clips.
Set the knob for the voice you actually record and leave a little travel for plosives, instead of pinning it because the spec sheet has a higher number. A booster on a chain that already has about 0 dB spare only adds a box. Most of those boxes also use the phantom power themselves, which will silence a condenser.