Fairy-Stockfish is a nice Stockfish fork that can handle a variety of Chess variants. Find and install it following the directions here.
Create a file called variants.ini. Paste the following into there.
maxRank = 6
maxFile = d
promotionRank = 6
startFen = rnbk/pppp/4/4/PPPP/RNBK w - - 0 1
Alternatively, download and save the linked file somewhere on your computer, which comes with the 4x6 variant along with several others.
You have a variety of choices for which front end to use. I am going to use Xboard. If you happen to be on Windows, you can use Winboard and the procedure should be fairly similar.
If you've chosen to install Xboard/Winboard, you can use the following procedure. Click engine on the menu bar up top and then go to edit engine list. Modify and add the following line
"Fairy-SF" /variant=4x6chess -fcp "/path/to/fairy-stockfish-binary load /path/to/variants.ini"
Here you should modify your /path/to/fairy-stockfish-binary to wherever that binary/executable exists and modify your /path/to/variants.ini to wherever you earlier created/saved your variants.ini file.
You can now go back to Engine in the menu bar and select Load New First Engine, selecting the engine you just added. You should now see something like the following image.
You can play the engine from this point if you want. Alternatively, you can paste in a position and ask the engine for analysis. To do the latter with your initial position, copy the following fen to your clipboard:
rnbk/3p/pp2/1P1P/2P1/RNBK w - - 0 1
On the Xboard/Winboard menu bar select Edit and then select Paste Position. The position should now be in Xboard. Finally you can select mode from the menu bar and select analysis mode. This should give you what you want: