Yes, as Mark says, she needs a more specialised milk, especially as she's not getting her mother's milk, but at least that Whiskas milk will have tided her over.
I've never reared a small motherless kitten either, but I've helped numerous mother cats with weaning. While your kitty isn't likely to be fully weaned at 4 weeks, she's quite likely to have started taking an interest in solids - especially meat. There are slow weaners and fast ones; some lie there placidly guzzling away at the milk bar while others are nuzzling at the mother's plate of food, uttering tiny 'mew-ew-ew!' s of possession, and slamming a threatening paw down (usually into the food) if another kitten approaches. Background can be an influence too; feral mothers, I'm told, tend to wean their kittens early; they'll dump a pigeon or mouse in front of the kittens and let them get on with it. Domestic mothers are, hopefully, provided with better food, and small dishes of special food for the kittens; they have more milk and can take it easy, whereas the ferals need to make their kittens independent as fast as they can; a nursing mother and kittens are vulnerable.
A good brand of kitten food, mushed up with a little warmish water might be a good idea to see if she takes any interest; braised chicken, boneless (of course) and skinless, finely chopped up, with a little of its own broth, and white fish (not anything oily like tuna) the same, served at room temperature, are suitable for a treat. Don't give her anything fatty, like the juice from a whole roasted chicken, which contains fat, and don't give her cow's milk; this can cause diarrohea.
I'd give her four tiny meals a day atm - say a dessertspoonful at each solid meal, but they're all different, you may find she wants more - or less. Off the top of my head I'd suggest 2 meat meals (if she'll eat solids) and 2 milk meals at the moment, but your vet and other people on here may have more advice. As she gets older though she'll clamour for meat and start leaving the milk.
When it comes to the other end of the kitten, it's amazing how the tiniest kittens know what a tray is for. Very young kittens need their backsides massaging (by the mother's tongue or in the case of motherless kits, damp cottonwool, in order to stimulate them, but at 4 weeks your kitten should be performing by herself.
Hope this helps for now - there will be other people along with more experience than me, and of course your vet will be able to advise and hopefully sell you the Cimicat/Royal Canin.
Good on you for rescuing her and hope all goes well
