Both 4G and 5G can use lower spectrum than 2G and 3G. The limitation is the following.
4G (band by frequency in Europe).
5G (band by frequency in Europe as of 2021).
700Mhz/800Mhz*/900Mhz/1800Mhz/2100Mhz/2600Mhz/3500Mhz/3700Mhz/26.50Ghz - 29.50Ghz*
*Not supported by every 5G phone on the market as of 2021 in Europe.
3G (band frequencies in Europe)
2G (band frequencies in Europe)
Lower spectrum use allows for better coverage but at lower speed. This is the same for 4G and 5G.
There are more frequencies that can be used but have not seen any use in 4G or 5G as of 2021 or allocation by regulators* and those frequencies are not supported by mobile phones either. Most phones support more frequencies then are used in any market for reasons of travel between Europe, North-America (ITU region 2) and ITU region 3 (Asia, Australia and more).
*The reason for this is that spectrum is being used by something else. Military, government, private communication by companies and so on. Fixed links communication and more.