I had to activate 4G on my SIM (with Virgin Mobile you dial 275 and press '2' to enable 4G on your service). Then I used the 3G/4G radio switcher app from openrepos.net to try out 4G (note phone/sms functionality does not work when 4G is enabled using this app). I also had to make sure I was using the 'yesinternet' APN (on Virgin Mobile). I'll probably stick to 3G for the time being until 4G is made available officially in a Sailfish OS update, but it is good to know it will work.