*maybe?*
Do g-parents have a vehicle with locking seatbelts (can be switched to locking mode by pulling all the way out?)
I'd consider getting a booster and doing a couple test runs around the block to see how he sits etc. It wouldn't be my preference to see a 4.5yr old in a booster, but if he fits it well and will sit still properly, for very occasional use it might be ok.
Most collisions happen close to home, so I think the distance travelled should probably be factored out of the decision if you're looking at it from a "not likely to have a collision" perspective. If you're looking at it from a "he can definitely sit perfectly still for 2 miles home" perspective, then I think it's reasonable if you get a booster that fits him well and he will sit properly in it.
I just want to caution you to not be considering the booster just because it's a short distance and the risk of a collision seems to be less likely.
Kids are usually mature enough/big enough around the age 5 or 6 to start using a booster periodically, and he is a good size and 4.5yrs old, so it's not unreasonable, considering that it wouldn't be on a regular basis...
So those are my thoughts. I can't tell you what to do, but hopefully some of those factors can help you decide what works best for him & what you're most comfortable with safety wise.
eta: I'm going to throw my usual disclaimer in here - I'm not of the camp that would harness a kid until the last possible moment if that kid was 7 yrs old or older. Kids reach a point where they're big enough & mature enough to use a booster part time, and then graduate to regular use of a booster. If your ds is at that point right now, and you're comfortable with it, it's not a cardinal sin to put him in a booster periodically. Just make sure it's a high back and I'd probably prefer one with good SIP. :thumbsup: