There is no best place. What works for one part of the country may not work for another part. You don't say where you are located, so I have no idea if you are City, Rural or geographically where you are.
Some of what I view as being the more popular Nanny recruitment websites are:
www.nannyjob.co.uk
www.childcare.co.uk
Gumtree.com has a section for nannies as well.
If you are wanting a live-out nanny, then advertising locally may work well - such as in a newsagents window, village magazine, local paper, other local advertising.
Nannies don't charge per child. In fact nannies don't charge at all... they are an employee so YOU set the price and they decide if they want to work for you or not.
Again what Salary you would be looking at offering will depend on your location. For example, London tends to be a higher cost than Home Counties. Midlands seems to be lower than Home Counties.
Salary you would want to offer may also depend on the person's experience. Given you want them to care for 4 children under 5, then you may be wanting someone with a lot of experience, especially of toddlers give there will be two toddlers.
A share can up the cost... as the nanny may feel a premium is required to help compensate for the inconveniences such as having two sets of parents to keep happy, two sets of parents to negotiate holiday with, managing possibly up to 4 different types of childcare voucher (if you, your dh, your friend, your friends dh, can all get vouchers from your work... plus if the nanny is registered to accept them).
In Home Counties I would expect you are looking at £10-£14 gross an hour. In London that could rise to £12-16 gross an hour. As a share I expect it will be towards the higher level of these ranges.
Have a look at other nanny job adverts for your area, that may help give you some idea of what other parents are suggesting they may offer someone... after-all someone looking to work with you will also be looking at other local nanny jobs.
With 4 under 5's... what happens about a car? Will you be supplying a car? As a nanny my car would not take 4 under 5's in the rear... the front seat would need to be used.
Doesn't matter if it's two children or 4 children... nannies are not paid per child... it's more a case per family. If it was just care for your children and not the other families, then salary may be around the lower level of the ranges given earlier.