Mike, be sure you separate the responses from people with a different engine than yours. Anecdotal sub-105C temp numbers from an OM617 are 100% useless to you (and me, and HGV). The OM60x are totally different animals and will run hotter than the old iron-head beasts. But I'm sure you already know this.
I'm surprised your car went up to 120C, that is quite high, for only a 4-mile stretch. What was the ambient temperature? Have you cleaned out your radiator AND condensor so they are bug- and debris-free? Otherwise, as I mentioned on the diesel list, use a bottle of WaterWetter (if you're not already), and if your fan clutch is original (metal blade) I'd seriously think about replacing it as a preventive/predictave maintenance item.
The rest of my previous email still stands, i.e. if the car never goes above 105C under ANY other conditions, I wouldn't worry too much. But I'd still check out the fan clutch. I'd love to hear your resuts if you do replace it. Maybe try "forcing" it on, like someone else did (wiring it to the pulley somehow)??
Good luck,