How seriously should I take the maximum pressure rating of my cyclocross rims? I own a very nice cyclocross bike that I use for daily commuting. It is currently fitted with relatively wide cyclocross tires (Continental Cyclocross Race 35-622) that I inflate to 4 bar (58 psi).

Since those tires are nearing the end of their lives and because I'm mostly riding on paved roads, I'm considering to switch to road tires (e.g. Continental Grand Prix 4000S II 28-622). However, I now noticed that my rims (DT Swiss XR 400) have a sticker saying "maximum tire pressure 4 bar" next to a pictogram showing a rim apparently broken at the weld. That pressure is of course hardly enough for any narrow road tires.

My question is now how seriously I should take this limit. I have a hard time imagining how too high pressure would break the rim and how exactly it would fail. Sheldon Brown mentions on his page that maximum pressure ratings are often rather arbitrary and he encourages experimenting, but he's talking about tires there, not rims.

I am willing to risk voiding the warranty on my rims or causing premature wear. But I would like to avoid the risk of catastrophic failure in the middle of traffic.
DT Swiss publishes the exact document you are looking for:

Manuals page / RIMS / Tire Pressure/Dimension (PDF)

The document specifies the maximum usable tire pressure based on rim and tire width. More narrow tires allow for higher pressures. For example, the rims in the question (XR 400, inner width 18 mm) are compatible with tires ranging from 23 mm width at 9.0 bar to 60 mm at 3.2 bar.