@ibonyun made a comment in the comments section to his answer. I need to answer with images which can not be done as a comment, besides there is also a need for this subject to be further sorted out.
LarsPeterSchultz I have never seen more than 3 slashes. Can you show me an example? It would certainly be non-standard. Yes, the 3 slashes can also mean 32nd notes, but that's rarely what is intended. Even at slow tempos, 99.9% of the time "as fast as possible" is what the composer wants.
You need more knowledge and/or information on this matter. I have seen tremolo with 4 slashes many times. The tempo does matter.
Here is an example from Dvorak's 9th symphony. This image is from the 1.st violin part. Note the 4 slashes for the tremolo in the slow part, Adagio, and the 3 slashes in the next part, Allegro Molto:
Sometimes there are discussions among musicians whether to play unmeasured or measured if the tempo allows a measured version. Note that among string players the term "tremolo" is applied in the meaning unmeasured tremolo, so it can happen that a composer write "tremolo" or "trem." in a string part in order to clarify that it should be unmeasured tremolo. Here is an image from Bruckner's symphony 5, it is the violin 2 part:
In percussion parts it is common to write Tr (trill) instead of slashes under the notes (although slashes can be used). "Tr" occurs often in timpany and snare drum parts. Here is an example from Mahler symphony 2, snare drum part:
And finally an image with a fraction from the solo piano part in Beethoven's 3rd piano concerto, second movement:
Because of a comment @ibonyun wrote below I find I need to make an elaboration to this answer as follows:
... To reiterate, 4 slashes is exceptional, as in non-standard, and 99% of the time 3 slashes means unmeasured tremolo...
You are correct that 3 slashes are much more common, but there is certainly nothing non-standard with 4 slashes. I am quite surprised by encountering that idea. As I said above I have seen tremolo with 4 slashes many times.
There are sometimes discussions amongst musicians, including conductors, on whether to play measured or unmeasured tremolo even when there are 4 slashes. It can also happen that a composer wants 4 slashes played measured although that is probably rare. Below I have posted two images with examples of tremolo with 4 slashes.
First an image from the book "The Cambridge Guide to Orchestration" by Ertuğrul Sevsay:
Second an image I made with four examples of 4 slashed tremolo written by different composers: