For the spec, the TBS6522 looks expensive to me for a dual card.
For usb dvb-t2 you are unlikely to hit a hub problem (assuming a powered hub). TV broadcasts are generally low birate (less the 10mbps, some uk channels get lower than 1mbps). USB2 is 480mbps. So, even if you assume the broadcast is extremely/unrealistically high quality (20mbps) then you could theoretically get around 24 recordings. Of course, there's usb overhead, but since you're aiming for four tuners, say two recordings per tuner, you should be fine.
As for DVB-T2 tuners, two popular ones are the xbox tuner (chipset MN88472), and Astrometa (chipset MN88473). I find the astrometa to be slightly better with my weak aerial, and has been supported for far longer than xbox; but xbox was half the price last time I checked.
If you're in UK, I think there are two DVB-T2 muxes, the rest are DVB-T. So, the other two tuners could be DVB-T if you find them cheaper. There used to be lots of dual usb tuners, but they don't seem so popular nowadays.
https://www.linuxtv.org/wiki/index.php/Hardware_device_information
https://www.linuxtv.org/wiki/index.php/DVB-T2_USB_Devices
I've read that usb tuners don't last as long as internal ones, but I've not had any problems. I think it's because in America they frequently only have "now+next" tv guide so have to constantly scan, causing cheap usb tuners to overheat.