You need to find out the size of the image circle. 4x5 needs at least a 160mm image circle. 8x10 is more than double that. The more the better because you want to be able to tilt and shift the front and rear standards and still have coverage. Websites like KEH and B&H will usually call out what the lens is suitable for.

Hi. I'm just starting out with 4x5 photography and need purchase a camera. I've rather buy a camera with lens but it appears to be easier to purchase a camera and then lens separately. How am I supposed to know if a particular lens is for 4x5 or if it will cover the film completely?