At least in the case of the #4820, I think it's an Atego MB chassis, even though the grill on the PM truck does look like it came from a Scania. However, the Scania is a double cab, and the Atego 1528 is a single cab with that drawer behind the cabin, just like on the #4820. Another fact is that being German, it would make more sense for Geobra to use a German brand like MB instead of Scania, and as far as I know (please correct me here) most big fire engines in Germany are from MB. Does somebody have official information that Geobra did use a Scania as the inspiration?
And what about the ambulance, police car and police bike? I did try to find the ambulance but I saw so many look-a-likes that could have been the basis for the #4221 that I gave up
.
Fire engines can have two types of cab, the normal truck cab which will seat two or three crew (including the driver) or what's known as a crew cab which will seat 6 to 10 depending on size and design. Normally in real life pumps (the 4821 is a pump) would have a crew cab but the size and cost of this rules it out for Playmobil. The first two pictures you show are of a pump without a crew cab and a rescue vehicle which usually only has the normal cab.
When it comes to turntable ladders, they usually only have the standard cab and carry a crew of two at most, but some brigades do have crew cabs on ladder trucks where these are fitted with a pump of their own as they can then act as a pump and/or a ladder unit.
Using German vehicles as a basis is what you would expect Playmobil to do and it is what they have done in the past. I expected that the new fire engines would be based on MAN vehicles and so it was a big surprise to me when the Scania type cab appeared. German brigades are moving to more foreign chassis nowadays, possibly because of tendering requirements within the EU which set out to prevent purchases based solely on the nationality of the vehicle. However, as VW have their eyes on Scania, it will all work out OK.

Looking back through the klickywelt posts on the new fire engines, the overall strong feeling is that they are Scanias.