Hi George, and welcome!
Let me say 2 things up front, my lawyer's disclaimers if you will. I can't speak about a Rachael from experience, and if you want to know anything absolutely positively for certain you should set up a call with Steve.
That said, I'm sure you can configure Rachaels as monoblocs. Look at page 6 in the manual and it will give you the details.
https://www.decwareproducts.com/_files/ugd/f1f204_778fc11e06344863a8011087bacad2...What I can speak about from experience is using UFO amps as monoblocs. And it is awesome. I started my Decware journey with a pair of UFOs that I configured as balanced monos. And they were so good I got back on the waiting list for a pair of UFO25s that I received about 6 months ago and I couldn't be happier.
There are dozens, maybe hundreds, of threads on this site by folks who aren't sure what amp to get. And the good news is you can't go wrong with any Decware amp. If the only thing that is holding you back from getting a pair of Rachaels is fear of failure when strapping them in mono then I would rest easy. Read the manual and if necessary talk to Steve and I'm sure you'll be fine. If you're happy with the power from you 4 watt amp then I would highly recommend a pair of UFOs. And if you have a balanced front end then I would recommend running them as balanced monos. To do that you need a custom XLR -> 2xRCA interconnect. Details on page 17 of the UFO manual:
https://www.decwareproducts.com/_files/ugd/f1f204_c93e1ffb7c8d472bbc314c7d4cdae4... The UFO2 and UFO25 all work the same.
Good luck, there are a lot of helpful people around here and you are likely to get plenty of other advice.