transfer time Athens airport to Piraeus?

I assume you mean a 4pm ferry FROM Piraeus to some island destination?

Assuming the plane comes in on time, you'd have to figure on at least another half hour to get off the plane, get your luggage, and get to the bus stop. If you do not have at least 1 and a half hours over by the time the bus leaves, then you are likely not to make it.

With everything working in your favour it will still take at least an hour to get to Piraeus by bus. And you'd still have to get to the ferry from the bus stop (walking time varies, depending on the ferry you want to take).

The other option is to take a taxi and ask to be taken direct to the ferry you want to take. You'd probably be able to buy your ticket from a kiosk by the ferry -- assuming there is space available etc etc.

Booking a ferry ticket in advance under these circumstances will be a risky business. You won't get a refund if you miss the ferry. All in all, it looks like a tight squeeze to me. With my own low tolerance for stress-related rushing around, I'd avoid trying to make such a tight connection. Stay in Athens/Piraeus overnight and take your ferry the next day, or fly out if that is an option and you are in a hurry.

Or just go for it. If you don't make it, you still have the option to stay in Athens/Piraeus overnight.

where are you heading? could there be other alternatives from Lavrio or Rafina?- the quickest way is via taxi to Piraeus, it should take about 45 mins,-
have a plan B.

Good advice guys, thanks
Yes, I did mean a ferry FROM Piraeus (to Sifnos, we are planning to book ahead)
I thought there was a Metro that would get us to the port with one changeover (acc to the LP there is anyway ...)?

not the metro! it will take double the time to Piraeus than the bus. Take the airport bus to Piraeus
Bus x96 to Pireus from airport terminal every 20 minutes.
or just take a taxi - cost will be 30 euro if you're cutting it short.

There is a New train service to Piraeus since 6.6.2007