There is no strict four-hour-rule but adequate access to daylight is indeed required.
Most rules are available at https://www.baua.de/DE/Angebote/Rechtstexte-und-Technische-R... and a good summary is at http://publikationen.dguv.de/dguv/pdf/10002/i-7007.pdf
I'm very interested, but my google-fu is not good enough apparently.