The 4PM has almost a same functionality as the 7PX but less channels in a less fancy housing and a less fancy screen. It supports the common S-FHSS, T-FHSS and superfast SR modes. You can co cheaper with the 3PV and 4PV that only do not support the SR mode.
Sanwa has the MT4/MT44 as a step in, I believe the MT44 also supports their superfast SSR mode.
Even both brands in normal digital servo mode are faster than the Spectrum.
Spektrum has never tried to make faster systems and with many AVC sets it looks like they are focussing more on the basher market.