You could do both, either use 2 separate 2x2 Mimo Antennas or use 1 Full Band 4x4 Mimo Antenna feed one pair (2x2) for 4G and the other pair (2x2) for 5G, but getting it to work correctly is the challenge.
Heres why, you need to do your Homework on your current Mobile Network surroundings -
1. Is the Network Transmitting the 5G signal from the same Mast as the 4G?, if its not, then using a Combined 4x4 Mimo Antenna isnt going to perform the same as using 2 lots of 2x2 Antennas pointing in different directions.
2. there is no such thing as a 5G External Antenna Ports, most 5G Router Ports only work around the 3.5Ghz Bands, so even if you connect a Full Band Antenna (600 to 3800Mhz) to your 5G Ports your not going to cover the 5G Band N28 or any Re Farmed LTE (3 and 4G) Bands for 5G NR Use.
3. There's a good chance that Adding another External LTE Ports to your Router, that your going to faced with a choice of Bands to use, probably either (600 to 1800Mhz) and (2100 to 2600Mhz) so again which Pair.
Cell ID Locking or Band Locking could help with the above scenarios, or make it worse, you really do need to do some Homework first on your Network Provider, then ideally build in some resilience should they Upgrade the Network or you change Providers.
In the deal world of current 4/5G NSA, MiMo and the way Bands are being Re Used for 5G NR, its always best to use Full Band 4/5G Antennas so your covered, but Full Band Antennas are hard to build and get right, its a very wide Spectrum, hence why 5G Routers break down the Internal Antennas into Bands (and External Ports), Mobile Phones are the same.