I do not think I have used the Benchmark LA4 with the older ST and SST series. I used to own the ST and SST amps. Those amps were hot on top. The 4B3 does not seem that way.
If the 4B3 is similar to the CODA #8, #16, Sanders Magtech, or the KRELL Duo175XD then I would use the LA4 as my first choice. I used it with all 4 amps, and it was the best preamp I had on it. I tried about 7 different preamps from $800 to $8000. I owned the old Bryston BP-25 and that was not good in comparison to the LA4.
I will be selling my LA4 next week. I traded my Benchmark AHB2 amp today for the Benchmark HPA4 preamp. It is identical to the LA4, but it has an additional headphone amp.
If you want more warmth, I am fan of the new Schitt Mjolnir v3 (own it). A class A preamp, but not as good to me as the LA4. Though no remote.
Another preamp that is great (and I own) is the Holo Audio Serene. It is almost identical to the LA4, just a bit warmer.
I think all the preamps I listed would be good with the 4B3. Infact, I was thinking of getting the 4B3 from my Magnepan LRS+ and use with the Serene or LA4. I decided to use the CODA #16 instead.
BTW - do not get fooled by the lower prices of the preamps I am suggesting. They are really good, if you do not want the preamp to add too much to the sound, except the Mjolnir v3 which adds a lot.