I have a bitch with 8 pups at 4 weeks old would it be ok to bring in another 8 week old pup from another litter

What is your reason for this?  It has always been said,  never have 2 puppies at the same time.  You need to devote your time and energy to the pup you already have, in training etc before having another.  If you go ahead, they will become 'best mates' and ignore you making life harder.
I've always said, when you are happy with the pup you have in terms of fitting in with your life, obedient etc then is the time to consider another.

Can you give a little more info please? Is there a reason why you need to introduce an 8 week old pup from another litter?

I have a bitch which I took a litter from her they are 4 weeks old there has been a chance of another pup of a colour that I've been after for ages and comes from great lines when it comes the pups I have will be nearly 5 weeks old all the pups I have will be all going

I don't see anything wrong with this, but would probably keep the 8 week old away from the 5 week olds.