Is it normal for one of a litter of four (4 week old tomorrow) not to be as active as the others? Ive checked it over and it seems physically OK but it only ventures out of the litter box (they've chosen to use as a bed!) for food and water and doesn't play like the other 3. The other 3 are bombing round the cage for ages before hopping back to the litter tray to toilet or nap but this one seems content to just snuggle up and sleep. I've tried lifting it out and placing it where the others are playing but it just hops back to bed or climbs into a nearer bed and just watches. It's gaining weight at the same rate as the others and toileting normally but it just seems odd...