So, the least expensive yet featured portable dock that doesn't increase the output cables of MBP and input cables to portable display that worked with my M4 Pro MBP was the one below. It was on sale when I bought it (~$39ish), but it's imperative the dock you get have a USB-C port designated for Video output - it has to say this explicitly, not any USB-C, no matter then Gen, will output video signal. More formally, the USB-C port on dock needs to have DP-Alt mode.
So, I plug the magsafe power cable into the MBP, then I connect the dock to one of the MBP's TB5 ports. Then I connect a 10Gbps or greater USB-C video/data cable from the portable display to the dock's USB-C Video port. Viola! The dock doesn't need a power adapter connected to it, it pulls enough from the MBP. This is similar to ToddGeorgeKelly's comment - having a dock in the middle "solves" the problem. But this works well for me, no issues.
So that's still one cable into the monitor and one from the MBP, if you consider the dock block a "cable". You can also use the dock's PD input for power; there is a Magsafe -> USB-C adapter in the wild, but I won't vet that.
Not as clean as a single, continuous cable from MBP to display, but still effectively one "cable" if you consider the dock part of the cable, and seeing that only one port is used on both the monitor and the MBP.
The dock (~$50): Inateck HDMI Docking Station 4K 60Hz, USB C Video+Data, 100W PD,10 in 1 USB 3.2 Type C Hub, 2 USB 3.0 A, RJ45 Ethernet, 1.6ft Cable, for Windows 7/10/11, Linux, MacOS 10.2, HB2030
The portable display (doesn't exist on Amazon anymore as a link, but it was only last year so maybe it changed name): CNBANAN 2K Portable Monitor, 14" Portable Screen IPS 2140x1440 QHD USB-C&HDMI 100% Adobe RGB Laptop Monitor Extender 1500:1 FreeSync Ultra Slim Extern
But, this is still not optimal, but a sensible work around if you don't want to buy another monitor.
Oddly enough, this monitor below worked when connected directly to my M4 Pro MBP, and it's older than the one I mentioned above, but it's too dim and the aspect ratio doesn't match my MBP. No clue why. Maybe required less power, who knows: https://www.amazon.com/gp/product/B09BB5T7FV/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1