Question is if I can enter Germany without a hotel-booking for those 4 weeks?
Yes (assuming the 4 weeks are within the year of your visa). You may be confusing authorisation to apply to enter the Schengen area (which you now have) with authorisation to travel around the Schengen area (which, with a few exceptions, everybody inside the Schengen area has).
In applying for a visa (for Spain) you satisfied the authorities that you would not be a drain on the Spanish state and, given that you were granted a multi-entry visa, not on other Schengen states either (even though Germany may well be considerably more expensive to live in than Spain!).
And/or would I need some kind of invitation, or guarantee confirmation letter from my friends stating that they will give me shelter, food, etc. for those 4 weeks?
No. Such considerations are relevant to deciding whether or not to grant you a visa. You already have a visa, so these are no longer relevant.
However, bear in mind days in Germany as well as Spain count towards the 90 days maximum allowed in any 180 day period.
Credit:stackoverflow.com‘
5 Mar, 2024
5 Mar, 2024