My Cavalier had 8 puppies on 30/9/13. Puppies weighed 7 and 8 ounces, however one little girl was 4 and a half ounces. At 5 days the other 7 puppies are huge compared to her, they have gained 3 and 4 ounces, she has only gained 1 ounce. She appears to be mobile and feeds regularly from Mum. I'm stumped as to know what to do for her? Supplement feeding? Have never had a puppy so small. If anyone has any suggestions of what to do, I really will try it

Rather  than looking at the actual weight gain look at the percentage increase and you will realise the small pup is not so far behind.

We have a litter at the moment, five pups with one being considerably smaller than the others.
The others were between 142g and 174g (5oz-6oz) but the little one was 93g/3oz. He has slowly gained weight pretty much daily, he has the odd day of no gain, but so long as this is just one day and he doesn't lose any then that's ok. Most days its a 10g gain.
He is now up to 202g/7oz at 15 days old.

He feeds from mum very well, is active and responsive. His eyes and ears are open now (he was a few days behind everyone else in this), and he is as vocal as the rest! I have just been making sure that he gets 'plugged in' every hour for the first week during the day and every other hour at night. He doesnt always want feeding but come the next meal he is ready for it. I'm just keeping him topped up.
I put him on before anyone else, he is strong enough to bring the milk down on his own. If yours is weaker then put another pup on the get it going before offering to the little one. I let him fill up, then let the others go on.
I also gave him beef liver remedy for the first week or so.

Another breeder in my breed had a similar pup, I asked for her experiences with her and she said that she behaved the same as mine, developed at a similar pace (though hers didn't open her eyes until 16 days), fed well etc. She has gone on to be a smaller but healthy bitch who weighs around 5.5-6kg, our breed average is 6.3-8.1 kgs.

I will be getting my boy checked by the vet for any obvious abnormalities on a regular basis and having a liver shunt test done when big enough.

I would personally just stick with keeping her well fed from mum, plug her in as often as possible! I know others do suppliment feed but I'm not a fan of this if mum is producing good milk. But that's just my opinion. You may wish to supplement, you go with what you feel is best for your pup :)