If you've done a BIAB you have an idea of how much you boil off in the hour. That amount will stay the same if you do a 3.5 gallon or if you do a 5 gallon. It's you, your stove, and the boil you have. It takes the same time to steep or mash regardless of how much you do, so even if you were doing 100 gallon batches it would take the same amount of time.
Oatmeal is flaked oats. I buy mine at the grocery store because I use them for other cooking. If I only wanted 4 ounces I would buy them from the LHBS.
Oh, if the recipe you are looking at has a 7 gallon boil for a 5 gallon batch (normal) you would reduce the amount of grains for the smaller batch, not double them.