Low 5's are good and sub 5's are great for the size. However, dash time is very low on the things that define a good lineman. You rarely run more than 10 yds per play as a lineman! What you need to focus on is the explosive quickness and agility. I'd take a big guy whose light on his feet with a quick first step over one with a 4.5 dash time any day.
Not even close. The kid is 220 lbs and out of shape. Running a 5 on most H.S. programs isn't fast enough to start at any position, unless you have tremendous size and strength. If you are talking about a 300 lb pro lineman, than around 5 maybe. Also, lineman run the forty just like everyone else. Both to get into college and at the pro combine. I ran a 4.7 my first year in college, as a 240 lb linebacker, and I also played guard in H.S., and I wasn't the fastest linebacker on the team. Offensive lineman have to be fast enough to block us, and big enough to block defensive tackles. It takes speed, agility and strength. A smaller H.S. player should be able to run under a 5 second forty no problem.
Explosive quickness and foot speed is important, but so is overall speed. Offensive lineman have to go downfield to block for screens and delays, along with sweeps, in front of running backs. Guys are getting faster all the time. Our biggest lineman in H.S. also ran a 4.9 40, and he was well over 300 lbs, and that was 25 years ago!