Obviously power will be less but tonally how would it compare to an SVT ( never owned the V4B)
Interesting questions; I'll answer as best I can.
To start with my SVT is '72/73 Magnavox vintage and the cabinet is a flip tops flatback cab with their Neo speakers (which are as close to the vintage square back SVT 10" speakers as I've heard - and I have heard the originals, albeit with 40 years of age and use on them).
Through the same cab the YBA100 is a more polite sounding amp with everything at noon. I like the deep switch as it seems to tighten up the sound and gives a great clean tone, without the deep switch engaged you get a much rounder tone. If you dime everything like I did (less treble and gain backed of a 1/4 turn) you get a great tone that is just starting to break up. When you increase the gain the break up in the YBA 100 seem to be lower in the mids, and not exactly my favourite, but for a good clean tube tone the YBA100 is great. The SVT comes at the cabinet with more authority because of the power under the hood and kicks a bit harder- the break up on the SVT is also more pleasing to my ears as it breaks up at higher frequencies. They're both great amps, and even comparing the YBA100 to a vintage SVT is a compliment, but they're different animals. For a reasonable volume rehearsal that needs a clean tube tone, the YBA100 would be great - if you band plays at reasonably adult volumes.
To sum it up in one sentence, the YBA100 is a great sounding, moderate volume clean tube head that tonally is in the family of the YBA 300. (Note I've never taken the YBA300 to breakup but I love the clean tone on the YBA300, perhaps even more than my SVT, perhaps..).
Anyhow, after playing it in the shop for 10 minutes, it followed me home and joined my collection if that says anything.
EDIT: If you're looking for an overdriven 100(ish) watt tone or love a dirty bass tone you owe it to yourself to check out a YBA3 or a Mono Block if you can find them. I almost always play with a clean tone, but those two amps are super fun to play with a nasty amount of overdrive.