Ron,
I messed around with it a bit to try to get a clean 3-beat ending for a 4/4 shuffle but ended up resorting to modifying the BIAB tracks in RealBand. I used the following four steps. A link to the result is provided below those steps. There might be a way to achieve the same result directly in BIAB but that's something for another day :-)

1. Set BIAB for a standard 2-bar ending with a 4-bar ending for RealTracks.

2. Create a BIAB song, but include 1 extra bar just before the ending bar with the same chord as the ending.

3. Import the BIAB song into RealBand.

4. In RealBand, delete the first two beats of the 1st automatic ending bar for all tracks. NOTE: If any of the tracks play something that is unwanted during the ending, delete two leading beats from them in some bar prior to the ending bar.

Link to the .wav file:
https://drive.google.com/drive/folders/1peiG3w3tqDNic_4Vi5TsecintvkWsJWC?usp=sharing