The difference of 4 watts vs. 100 watts is actually close to 2.5 S-Units, and then another to 500 watts for a total of 3.5 S-Units or 21db.
I'm not sure of any mobile antenna worthy of a 21db gain.
Either way. 2.5 S-units difference is barely enough to make a difference on the person's end. you're going from 36 dBm to 50 dBm. It's barely noticeable.
But let's look at what I'm trying to say: For all the work entailed in putting an amp into a vehicle (Which often will need an alternator upgrade if it's a smaller vehicle), plus heavier guage wiring needed to support the power; when one can just put a more efficient antenna, and better coax.
And, one can gain 21db of signal, merely by getting better coax,.
And, one can usually get better returns by simply improving their ground plane.
Not to mention, the 4W to 100W really isn't even accurate. CB radios have a legal limit of 4W carrier. On a 100W amp, you'll only be getting a 25W carrier. If you're using SSB, you have 12W PEP, compared to 100W PEP. Now, you're only looking at 40 dBm getting bumped up to 50dBm. That's barely an S-unit and a half.
Now, 100W PEP to 1000W PEP is only another 10 dBm. Again, barely an S Unit and a half!
An amplifier would be the last thing I would look at getting to improve a station. Antenna, antenna, antenna. After you have the best gorram antenna you can get, then it's: Coax, coax, coax.
After you have the best coax, and the best antenna, then you look at consider an amp. The amp does no good if it's only cooking worms, or cooking your lossy coax.