
In toy breeds with ones and two's and high mortality in pups I think in some circumstances you might have to go over four to get your pup.
I think the 4 litters as in my breed is fair enough as it allows for the odd occasion where litters are small.
I have had up to three litters from any of my girls so far with a maximum of 18 pups born to any bitch. One had just two litters with 8 pups in total.
If I had huge litters I might choose to have fewer, but if I had say two litters with small litters and a great opportunity to use a particular male might stretch to a fourth. A lot depends on the fitness and age of the bitch.
I generally don't breed until around 3 years of age, so there wouldn't usually be time for more than three litters, with sufficient rests between.
My Jozi a top winning bitch with Champ show Group placings and a champ stakes win had her first litter at nearly four, her second nearly two years later, and her last about 18 months after that a month after her 7th birthday. I used two different males, one a US import the other a Norwegian one so that I could use her offspring from both in future breeding plans.