I have an old Dell OptiPlex with 4 RAM slots. It's max RAM is 8GB. Does this make each slot max 2GB or can I put a 4GB stick in slot 1 and a second 4GB stick in slot 3?
That depends on the particular motherboard. Either one could be correct. Have you got a more specific model number? Or can you read the model number on the motherboard?
Actually Optiplex 745 and 755 can work with 16GB if RAM is compatible, like Crucial, Corsair and Kingston.
Position correct 1-3, 2-4, where is 1 next to CPU.