Hi Thomas (and all), I've just switched to my FT4 config, toggled Tx4 to RRR instead of RR73 and then closed and restarted WSJT-X in the FT4 config.
I've found that Tx4 does indeed get reset to RR73 as you'd described, but as Frode LA6VQ has pointed out it's a trivial task to then toggle Tx4 back to RRR. If this is an inconvenience for you then you might need to review why you need to restart WSJT-X after every QSO... :-)
