I'm not sure what SYDT means, but assuming you're referring to the Sydney International Airport (T1), I would recommend arriving around 1:00pm for this time of day (2 hours before departure), as a base case if you're walking to the terminal from the Rydges and have no risk of transport delays. If you're driving or taking public transport, my recommendation is to add twice as much time as the expected transport time as a buffer, i.e. if your train will take an hour, leave 2 hours earlier for the above arrival time (i.e. 12:00pm for the 4pm flight).
This is a quiet time of day for the International terminal. If your flight was a bit earlier, say 2pm or earlier departure, I would set my arrival target a bit earlier (the busy time of day is morning through midday).