Spotify Customer Support are super nice and friendly and can look into this for you. You can get in touch with them through the online form or if you have a Twitter account, feel free to tweet @SpotifyCares at anytime 🙂
That shouldn't be the case. If the only option you have is me + 4 it means you're already on the Spotify Family plan with 4 subaccounts. Can you double check that info from here and see if this is the case?
If you upgrade from Premium to Spotify Family, your remaining subscription time will be shared between your subaccounts. So most likely you'll be charged for the Spotify Family rate before the usual renewal/due date.
Spotify Customer Support are more than happy to help you on this. I recommend that you get in touch with them through the online form, or if you have a Twitter account, feel free to tweet @SpotifyCares at anytime 🙂