the 6 yr old yes, the 4 yr old would depend on the child and the circumstances, i live in a quiet cul de sac and all the kids play out, my 8 yr old is out at the moment and with the sunny days over easter my 6yr old was out as well.
BUT i can look out of my front living rm window or kitchen window and see them and i nip my head out the front door and check on them every so often, i let my 3yr old play out in the front driveway but it is gated and fenced in, and he knows not to go out in the cul de sac. he needs supervision ie i sit out on the bench in the front garden/driveway as he scooters up and down.
i have set limits for mine ie they can play out the front of our house and the ones to the right but they are to go no further than a certain lampost. it works for us :)
previously we lived on a busier road and there was NO way i would let them out.
my 11 yr old has in the last year started going into town (20min walk) on his own to the cinema etc and he goes to the local park.
oh and we have a shop 3mins walk from our house, i now let the 8 yr old go there on his own, ie to buy a packet of match attax cards, he knows to go straight there and back adn only has to cross two small, quiet roads, we live on a housing estate.
it IS hard letting them do these things but my eldest started high school 2 wks after his 11th birthday and you do have to let them start having freedom in preparation for high school etc. but it is ALL dependent on where you live and most importantly the traffic around where you live.