a.Find all logged-in users with usernames of at least 4 characters

i dont know how to make this at least 4 chars thing
-name "****"
but i think its not correct

b.Find all regular files in /var directory that were changed more than 4 days ago and has size more than 1 Mb

i dont know how to define changed because
there two kinds -mtime and there is -ctime

and the last problem i dont have a clue how to build it

c. Use tr command to substitute all lower-case letters for upper-case ones and vice versa, i.e. upper-case to lower-case. Use manual or any other source to learn the tr command