Was short hand for 4 to 5 oz.
And that’s light weight. Most coloured Dacron sailcloth eg contender coloured - is 4oz / sailmakers yard
White plain weave I’ve used in thruster, flightstar and a couple of others I made sails for were either 4 or 4.4oz.
The leading edges on the weight shift sails I’ve made are all 8oz with a 4oz outer colour sleeve.
And for clarification a sailmakers yard is not 9ft^2 but 36”x28.5”
So doing a bit of conversion 4oz Dacron weighs around 172g/m^2