A difference in circumference is the norm when using a space saver spare wheel. You are of course speed limited while running the saver, and it is definitely best to fit it on the non powered axle, even if that involves shuffling the wheels to achieve that.
If you're running 225 R18s on your regular wheels I'm not surprised you're unenthusiastic about getting a fifth matching wheel. In your position, I'd put greater emphasis on buying premium quality tyres and on replacing them sooner rather than later, with an aerosol in the boot for emergency use rather than faff about with a space saver.