Note: Number of tuners = number of frequencies you can access, not number of services. With a single terrestrial tuner, you can record all the services on the frequency you are locked onto.
The SF408 is a 4K receiver, but you can only use three of the internal tuners (Even if you have two twin-tuner modules). Not sure if it will be able to use additional USB tuners.
The ET8500 is the only other 4 tuner slot receiver. May not be your cup of tea if you don't like the front display.
Pity there are no FBC terrestrail tuners.