The compile time is 2.8sec (taken off the compile and rapid debug form). You need a large enough project, something that compiles for longer than 1 sec.
B4J IDE needs to be in debug mode, running. I'm using an i5-8400T 17.Ghz, 12Gb. 64bit W10
C:\Program Files\Java\jdk-14.0.1\bin\javac.exe
Hit Save button, wait at least a half second, but less than 0.75 second, then hit save again. Try varying the time delay a bit between 0.5 and <0.75 sec. Times may vary with project size.
After about a second, despite IDE still says compiling, it won't reproduce. Maybe it has started the hot swap but didn't update the status bar so it appears to be compiling.
I restarted the IDE, hit F5 to run, then hit the save button twice with the delay as described, and it reproduces.
I installed the Release. Still reproduces.