I currently have dot 4 brake fluid in the system but have acquired some calipers that had dot 5, can I flush them through with dot 4 or will that not work? what is the easiest/cheapest way to use them?

If it was Dot 5.1 then you can do what you suggested, they are both based on glycols. Dot 5 is silicone based, you would need to strip, thoroughly clean and ideally replace the seals.

have only ever used dot 5 on my karts....5.1 is more than good enough for car brakes...