Weight has been far more accurate than volume for me. I use a scale for everything and I weigh everything in grams/ML because that is more accurate and makes more sense
Once you are allowed solids, it is unlikely you can eat 4 ounces of it at once. It took me months to work up to it. The exception was soft baked fish, but that is really a soft food, not a solid.
Even at 2 years, 4 ounces of solid food will still vary for me based on moisture content and the mood of my sleeve that day.
You don't have a book that tells you what you should be eating? I was allowed 2 ounces at first and even that was a stretch.
Also, this really isn't a question you should have to ask online or be confused about. You paid a team of people for this surgery, they should be telling you explicitly what you should and should not be doing. Make them.
Weighing your food long term is going to save you a lot of calories over measuring. In the beginning a WLS patient that is healing can't eat much, so it doesn't seem like this matters much. Long term, when you are stalled or can't seem to lose and you think your calories X they might be Y. These small calorie differene over the course of a day, week, month and year add up. Better to create strong good habits in the beginning and carry them on long term.