Actually 4G when using LTE-Advanced technologies, such as Carrier Aggregation (harnessing multiple bands at the same time for a single connection), is designed to cope with speeds up to around a Gigabit to end-users. Of course you rarely see this in the real-world due to limited capacity, costs, signal variance, your choice of mobile plan and hardware limitations etc.
For example, EE has been able to deliver 400Mbps+ in some urban areas over 4G for the past few years. Vodafone are catching up with that sort of peak performance, while O2 and Three UK are some-way further behind. Not all networks and devices are at the same level of development.
As for mobile "compatibility"... you could say that all mobile devices are backwards compatible because they support all of the standards, although each 'Generation' is still somewhat independent in how it works / is managed. Your mobile device generally chooses the best available service / signal (3G, 4G etc.) and connects to that. The mobile network itself also aggregates all of these communications so that it doesn't matter how you connect.