Take your lumen output per square meter and convert to average ppfd( ~.0145 for our 3500k Crees)
4000 lumens/sq ft * 10.7 (sq ft per sq meter) * .0145 = average ppfd of 620/sq ft
Or I guess you could combine the 2 factors and use .155 per square foot.
4000 lumens * .155 = 620
Par watts is the other conversion method. Lumen output is just usually quicker to find.