I have a four year old and find both the sight of a four year old in a buggy drinking from a bottle, and the idea (haven't seen it for quite a while) of a four year old pulling up her mum's top and helping herself personally distasteful. That's not quite the right word, I'm trying to think of something less forceful than repugnant, but showing that I'm uncomfortable with it.
That's my gut reaction, and not something I would show, or say to the mother. Thes reason I think I feel this way? Partly it's because I believe that a four year old is not a baby, and so shouldn't be doing a baby thing (feel the same way about dummies).
Partly it's because I don't like to see children get what they demand, and I think it's a huge invasion of space to go to someone's breast (even their mum!)unless specifically asked - which I do think would be a bit weird!
But then, I didn't enjoy breastfeeding very much because of the personal space issue, and feel that once weaning the whole thing gets so messy, and once teeth painful!
I would not censor, or feel that I have any right to censor, anyone who choses to breastfeed for however long they wish. But I also don't feel that I should feel bad that I feel uncomfortable.
Also I do think that the fact that mothers in some parts of the world breastfeed for a long time is related to the safety of water supplies. I've not seen any evidence (not that I have looked really) that shows that when there is no risk from water supplies there is any benefit to extended breastfeeding.