Wikipedia has this on the "Formelsammlung" page:
Eine Formelsammlung ist ein Nachschlagewerk, das fachgebietsbezogen meist naturwissenschaftliche oder mathematische Formeln enthält.1 Es werden in Formelsammlungen in der Regel keine näheren Erklärungen bzw. Beweise dargestellt. In Form eines Buchs oder einer Broschüre dient die Formelsammlung unter anderem als (zugelassenes) Hilfsmittel in Prüfungen oder als Lernmittel im Unterricht.
Translated by my favourite AI automated statistics-based translator this means:
A collection of formulas is a reference work that usually contains scientific or mathematical formulas related to a specific subject. As a rule, no detailed explanations or proofs are presented in collections of formulas. In the form of a book or a brochure, the formulary serves, among other things, as an (approved) aid in examinations or as a learning tool in class.
This seems pretty straight-forward to me, and matches my experience in the past. In school (Bavaria) we had a little blue Formelsammlung IIRC, and it was exactly as in the quote. Lots of formulaeformulas, no explanations. Of course there were words and sentences to structure the formulaeformulas where it made sense, or to explain something about the syntax.
I cannot remember to be allowed to use a Formelsammlung at Uni, but I would go with that. Cram your 4 pages full with formulae;formulas; use titles or little intro words as necessary, and you should be quite fine.
Check out this Formelsammlungen Sekundarstufe I - Bayern - Realschule: Mathematik - Physik - Chemie - Formelsammlung - LehrplanPLUS and use the "Blick ins Buch" feature to read some of the pages. If your prof gives you trouble for your four pages, and if worst can comecomes to worst, you can show him a book like this. There are plenty of words, sentences, explanations etc.
And to elaborate on what you should not write:
- No proofs.
- No elaborate explanations (like a whole paragraph with free text on how to use the formulaeformulas, or with the "algorithm" how to solve problem X in general).
Here are more examples: