Is not pretty. It will take you on average 6 days to find a share. That share will be worth enough to cover 6 days of earnings, except the pool also needs to find a block while your share is active.
Some miners with 180GH of hash power don't like using p2pool because of the variance. Using it with 4GH... you can do it, but you might wait weeks at a time without a payment if you have bad luck.
For now, waiting weeks is unlikely (but possible). The average for the pool to find a block is only a day and a half, so very likely it will find one during the three days your share is active.
My feeling on this has always been if its a hobby (and it damn well better be a hobby at 4 GH) then treat it as such, have fun with the small "raffle" and help the bitcoin and p2pool networks by running a node
This will get worse as difficulty continues to grow if the p2pool network doesn't keep up (and it doesn't look like it will), but still no reason not to treat it like a fun raffle if you are mining as a hobby.