Sleeping bag for backpacking in Vancouver Island

Ok so since the jacket discussion came to a closure I have another problem. Sleeping bag. I just bought a new 70l Osprey backpack. I have a tent and still need a mat and sleeping bag. The weather forecast says that the coldest temperature for the next few weeks is gonna be around 3-4°. The problem (besides the cost) is that the sleeping bags with -7° for example are too big and thick. I wont be able to put them in my backpack. Maybe I can let it hang outside but then Id wait to buy one in Victoria since its gonna cause problems at the airport.

0° bags are much lighter and smaller. Would you say that 0° (+tent, layered clothes and mat) is enough for those temperatures?