I'm sorry to be pedantic but you can't be using your box for 'Freeview' because that service is only available on terrestrial TV. So that leaves me wondering what you are watching. There are two possibilities, you could either just be watching the (unencrypted) 'free to air' (FTA) channels or you could also have access to the (encrypted) 'free to view' (FTV) channels, which are available under the banner of 'FreeSat'
You can receive FTA channels using a Sky box without a valid viewing card. However, Channels 4 and 5 are currently encrypted FTV programmes, so you can't view them unless you've got a suitable viewing card. I assume that this is the cause of your problem.
You can buy a FreeSat card, from Sky, for a one-off payment of �20 (i.e. with no subscription). If you buy the card, you'll be able to view more channels (i.e. you'll be able to watch the FTV channels). The full line-up on FreeSat (which is completely separate from Freeview) is here:
http://www.freesatfromsky.co.uk/?pID=3
Chris