Amazon - DVDs like Bali, Mila raconte ..., 5 Rue Sesame, Peppa Pig (VF), and so forth. Or buy them in France before you leave.
Books - ecloe des loisirs run several book clubs that deliver to Britain.
Magazines - Bayard jeunesse.
If you are in Edinburgh or near enough then there is the mediatheque of the Institut Francais too.
At 4, your child will still be blissfully uncool re music, I expect, so you can make up CDs or tapes, not just of nursery rhymes, but "classics" like Claude Francois, Johnny, Jo Dassin, Sacha, Les Surfs, Aznavour, and newer stuff like Zebda, Tete etc - my 3 year old loves "dancing" (her conception of dancing, not mine) to this sort of stuff.
If you get Sky or Virgin then, on weekend mornings, TV5 (Virgin channel 825) has a couple of hours a day of kid friendly viewing in French, from Le Petit Nicolas to C'est Pas Sorcier and a sort of Canadian Byker Grove (Taktik).
In London there are a few French language playgroups and Saturday schools for native speakers - perhaps there's the same thing in Scotland?
And, of course, visits whenever you can.
It can get dear and it's far from fuss-free - but it can be done!