File I/O functions, such as fscanf and fprintf, use automatic character set detection and UTF-8 encoding by default
Behavior change
As of R2020a, character-oriented file I/O functions such as fscanf, fgets, and fgetl trigger automatic character set detection when reading a file that was opened using fopen without a specified encoding.
Similarly, fprintf defaults to using UTF-8 encoding when writing a file that was opened using fopen without a specified encoding
You probably have a large file, and fread() is having to scan through a lot of the file to try to figure out what the character encoding is.
I would suggest that better code for you would be
beg4c = fread(fid, [1 4], 'uint8=>char');
