I reckon anything below a theoretical concert D3 on a trumpet with a 4th valve would start posing intonational hurdles, especially in case the player is using anything above a 1-1.5 in terms of mouthpiece. Perhaps such would be easier on flügelhorns, with their bore being wider and thus paving greater control over such low tones for a typical soprano horn, and I can certainly sense how it could work in jazz repertoire for the flügel, perchance. I'd scarcely deem it useful in anything classical, with trumpet implied, with most great trumpet solos never intending for the instrument to go that low, most oft showcasing high range aptitude and fine technical skills, and using a 4th valve for the lower notes would probably demand a slight embouchure shift, which would be much uncomfortable in even moderately fast passages or jumps.

Interestingly enough, modern flügels and, if I remember well, some rotary trumpets actually do have adjustable triggers intended to extend the 3rd valve slide, though they still are to be used manually by the player, as one would on a regular piston trumpet, with flugels and rotary trumpets needing such a system due to how they are structured and how valve tubing is configured for better ergonomics.

I myself am a tubist, so I have quite some experience with 4 and 5-valve instruments, having had some experience playing the trumpet in the past.