Generally Flex fares are MUCH better for mileage runs - you earn a LOT more miles than Light for not much more money and it allows you the chance to voucher-upgrade, even in the lowest booking classes.

As for your 4 flights - some of the longer leisure-heave route from ATH are usually cheap. Look at LIS or MAD (although I haven't checked recently). You'd have to buy the tickets individually though. I've had very good deals on ATH-LIS, ATH-DUB and ATH-EDI in business class in the past (LOTS of miles) although the latter two mean a lot of doubling back on yourself.

Also look at the Aegean Pass. The Balkan one seems pretty good value - especially when you book 30 days in advance. You could do three return trips from ATH to BEG, OTP, SOF, TIA etc. and they ALL earn full Y class mileage (as if you booked Flex). Then you could probably fly back to Canada via IST or something, without having to go back to Heathrow.