Good to see Osprey getting a few mentions here; I think they are superb bags. I commute with an Osprey Synchro 20. This is a cycling specific bag with a frame which holds the pack away from your back so that you have an airflow over your back. I commute about 18 miles with a laptop that weighs just over 2kg, plus some items of clothing, lunch and snacks in it as well as some odds and ends. I should imagine the combined weight is something over 4kg, but it doesn't cause me any discomfort. Indeed, I find I stop noticing after the first few miles. You may have a different experience, of course. I have a friend who would rather use panniers, so each to their own.