My guinea pigs are fighting?

Hi, I have four male guinea pigs. They have been homed indoors in pairs in separate cages. I have today put them outside in a large double decker hutch, and they are all fighting. Mainly the two that have been homed together, they never fought when in the indoor cage. I noticed that one of them has some blood on it's cheek, it has had a little nip. Should I persevere keeping them together or do I need to split them back up, and I was wondering why the two that had been homed together are fighting each other. I have been monitoring them all day and they are making a bird type noise, they are all asleep separately and calm at the minute. Any help would be appreciated. Many thanks.