I read somehwere that ledger nano only lets you have 4 different coins in there at one time, surely that is not true?
The ledger nano s lets you store up to 24 currencies at the moment. You can find out which those are here:
https://ledger.zendesk.com/hc/en-us/articles/115005304449-Which-altcoins-and-tokens-are-supported-by-the-Ledger-products-While it is true that there can only be up to 4 apps installed on it at the same time, you can just deinstall an app and install another one within 20-30 seconds.
The coins on the nano s stay safe during the whole process. They are all derivated from the seed. There is just not enough memory on the device to have more than 4 apps stored.