One key argument for 5G is that it supports vastly more bandwidth than does 4G. In fact, the practical argument for 5G is that it allows mobile operators to supply capacity at lower cost per bit, a necessary requirement to maintain profit margins as mobile operators must supply ever-greater consumption allowances at roughly fixed prices.
5G is, by design, capable of 10 to 20 times the downstream speeds of 4G. But it is not intuitive that 5G supports an order of magnitude more bandwidth than 4G. Could we not increase bandwidth just as much by allocating more spectrum for 4G?
In other words, since spectrum and capacity are directly related, why not simply run 4G in millimeter wave bands? The easy answer is that each next-generation mobile network takes advantage of improvements in coding efficiency and modulation complexity that were not possible in the older generation network.
In principle, even if 4G were simply extended to new bands, the new network would forfeit the 10-fold to 100-fold advantages in bandwidth efficiency and latency reduction. New networks also are designed to use wider channels, which additionally increases bandwidth within each channel.
Another answer is that 5G uses much-wider channels than older mobile generations. Long Term Evolution (4G) supports channels up to 20 MHz. 5G NR, on the other hand, supports channels featuring many hundreds of MegaHertz. As with all communication systems, wider channels mean more potential bandwidth.
Also, wider channels also mean less bandwidth “wasted” in the form of guard bands.
Ignore for the moment the practical reality that chipsets and radios are not designed to support 4G in millimeter wave frequencies. As a practical matter, new next-generation networks always are intentionally designed to work in new spectrum bands, to allow coexistence with older networks. That sort of makes moot the argument that 4G could be supported in new spectrum bands including millimeter and mid-band frequency ranges.
Still, the main reason why 5G is needed is that it performs better than 4G in terms of device density, coding efficiency and bandwidth.