Up to now I have always used FT8 rather than FT4, especially with QRP. FT4 is quicker, but not as sensitive as FT8. I can imagine with Es out of season, FT4 might catch some openings that FT8 might miss.
This would apply for other types of propagation too where very brief openings might occur. I can imagine some F2 DX openings might be very fleeting. FT4 might catch these. I am not sure how many monitor FT4, but I imagine far fewer than FT8.
One useful future WSJT-X enhancement would be automatic detection of FT8 and FT4 speeds so stations could use either modes in either slots.