Sounds like age restrictions have been set on the box - unfortunately you need the pin to take them off!
Try getting C4 this way
Go to Add Channels (services 4 4)
10729V
22000 5/6
Add the channel 8350 - this is a fta version of Channel 4
You can then watch it using Other Channels (services 6 on a normal box services 9 on a sky+ box)
Theres no fta versaion of C5 yet.