Is he responding to you? Try talking to him, do his eyes move, does he move his limbs? Is his breathing normal? Is he looking at theings, or just staring into distance?
Pick him up, is he holding himself up or is he floppy? Does he hold his head up?
If the answers above are no, take him to A & E straight away, if you're on your own or it's a distance, then consider an ambulance. If it's all normal no doctor will be upset, they've met worried parents before.
If the answers are yes then is he ill?:
Has he other signs of illness. Has he fed normally today? (if he's fed less he'll wee less) Is his temperature normal? (check back of the neck or tummy) Has he just had his 4 month jabs?
Sometimes they do go a long time between wee, then may do half a dozen in a short length of time. If he's breast fed then they don't poo everyday, my dd2 used to be 7 days between going. They do sometimes lie very still, often watching something intently. And sometimes, they suddenly have a much longer sleep than usual for no apparent reason.
Hope he's okay. It's hard to tell without seeing him whether it's normal or not. Trust your instincts, he should react to you.