I'd use the recipe they have on their site for pork shoulder as a guideline, since it's very similar in terms of size and shape to a beef roast (just not as heavy).
They estimate 10 minutes per pound, which is more or less consistent with the per-side estimate of most broiler recipes (which is very similar to infrared cooking).
So start with an estimate of 40-50 minutes but do what they recommend and monitor the temperature very carefully.
I have to point out that 4-5 pounds is very heavy/large for high-heat cooking methods like broiling or grilling (or infrared); it's not the same as infrared "frying" a turkey or a leg of lamb which has a very high surface area/weight ratio. Steaks would no doubt come out great this way but larger roasts respond better to roasting, so don't be too surprised if your roast starts to get seriously charred on the outside before it's sufficiently cooked on the inside.
You might want to consider starting with smaller, 2-3 lb roasts, and move up the weight next time if those come out perfectly. Based on their claims, it should take roughly the same amount of time for one large item vs. two smaller items anyway.