4G is a single "pipe" as it were, and everyone trying to use data on the O2 network in your area, are all trying to force it through that single pipe.
4G+ takes that single pipe and turns it into 2, 3 or even 4 pipes, all much, much wider than the original 4G pipe so everyone has a lot more space for their data, they don't have to squeeze it through anymore.
If you imagine a concert and people leaving at the end. 4G is all the concert goers leaving through a single exit, 4G+ is every exit being open, (or more at least).
If you're a long way from the mast you won't get 4G+ whatever you do, (which is why I recommended trying before buying), but if you're close enough and your mast is 4G+ enabled, its almost guaranteed that you'll see a significant speed bump, (I see 70-80Mbps on giffgaff).