I'm not load balancing ADSL and cable, but for something like that you might want to have a look at shared band and/or Andrews and Arnold ISP.
As for ADSL and 4G, I am doing that (well, whilst my 4G allowance remains). I'm testing a device that sends most of the traffic out via BT, but when that isn't fast enough it sends the extra bit by 4G. It uses my phone's 4G, but can also support MiFi devices and some makes of dongle.
BT ADSL customer getting 1.7 Mbps (0.6 Mbps up) on a new road / new build development
CAB not FTTC enabled, not part of the 66% commercial plan. Not rural - no BDUK funding
(Virgin Media nearby)