I'm pregnant (39+6!) with my first baby (gender currently unknown) so this post comes with my regular proviso that I may change my mind once I actually have to deal with all this stuff.
4 years old seems too young to me. But then, I also wonder whether my 4 year old would also be going to playdates without me unless it was with family or close friends that I could trust to respect my parenting decisions after an issue arose. So maybe I don't have a clear idea of what having a 4 year old is like at the moment.
I can see the argument that like face painting and dress up that its just a bit of fun about being colourful. But that argument could also be used for other makeup like lipstick and eyeshadow - nail varnish is a makeup, that's how it is sold and marketed. It is part of the culture of getting girls/women to groom themselves and value their looks above their other attributes. It could also restrict other play activities if the child is encouraged not to chip their nail polish and be precious about it.
Of course its occasional use for play with rainbow colours or to match a costume for halloween kind of subverts the prettification aspect of nail polish. That's also the context in which boys are playing with it too, as a bit of a laugh, its not something they are expected to do as an adult to be attractive. Just like having your face painted as an animal isn't the same as putting on makeup like an adult, even though its a similar activity and may even involve the same kinds of products.
In the grand scheme of sterotyping girls this is perhaps a small thing, and I don't think I'd be too judgey about parents that do it. I know they are just reacting to other girls doing it and the products marketed at young girls, it just seems a bit of a fun activity for children to do. But for me, its an activity I associate with older age groups, not little kids, so I can't see me being comfortable with my child (male or female) playing with nail polish at 4. The only expection I'd maybe make would be at halloween/other major dress up occasions where facepainting might also be involved, but even then, I'm not sure I'd do it from 4.
I don't wear makeup (including nail varnish) myself. I agree that these products often contain nasties that ideally we just wouldn't be putting near our skin. Of course there are other sources of toxins, but those in cosmetics seem particuarly unnecessary and avoidable.
Definitely use non-toxic nail varnish for children who are likely to put their fingers in their mouths if you are going to use it at all though. Just like you'd make sure that you use facepaints that are easy to remove too.
Nail painting (or even face painting) is not something that I'd immediately assume was okay to do to a child I was taking care of, whether they were female or male, 4 years old or 9 years old. I mean, its messy, their parent may not like it, the child may be allergic or the family/school may have plans for which rainbow painted nails or faces are inappropriate (photos/weddings/church/whatever) coming up. I also wouldn't do temporary tattoos or hair colouring for the same reason. (Or really messy painting/mud related activities without prior arrangement, don't want to get a kid messed up just before an important family dinner at a restraunt or something).
I'm glad that I have mumsnet to let me know that others would allow nail varnish though, so I know to specify that its not okay for someone taking care of my kids to do!
Mother to be to a PFB obviously! 