My main characters first appear in the book all at once. How do write their descriptions so that the reader isn't overloaded with so much information all at once?
It is not uncommon that a group of characters are introduced all at once. For example, C. S. Lewis' The Lion, the Witch and the Wardrobe begins with the sentence:
Once there were four children whose names were Peter, Susan, Edmund and Lucy.
The first paragraph does not distinguish between the characters and speaks of them in the plural instead, relating how "they were sent to the house of an old Professor" and that "they liked him" when "he came out to met them".
This is appropriate when the differences between the charactes aren't yet important and the focus is on their shared fate. Differences are introduced as the roles for the four children begin to differ.
Best practice is when the (main) characters of a work of fiction are clearly distinct.
Think of the silhouettes of the cast in an animated movie (borrowed from here):
In fiction, where your readers cannot see your characters:
Give them names that are written and sound differently.
As the names are the handles that the reader uses to recognize characters, make each of these labels unique and avoid similar names. A common practice is having each name begin with a different letter (not Ralph and Roger), but be careful of any alliteration (Mary, Karen, and Harriet) or rhyme (Rob and Bob and Tobby) too.
Give each character a distinct characteristic.
This can be looks (blond, brown, red, and black hair), clothing (red shirt, blue socks, etc.), features (angular, round, long face; slim, tall, round body), or behaviors (scratching, tapping one's feet), interests (football, chess, programming, history), traits (courageous, agressive, gentle), etc.
The characteristic you choose should be the fundamental defining trait of that character or clearly represent it (e.g. you might use large round glasses to signify "innocence" and have the character push them up, clean them, take them off etc.).
The four characters don't all have to be given the same type of characteristic, e.g. you can combine the blonde man, with the man with the angular face, the old man, and the nerd.
When you introduce your characters,
- label each one by their name and
- give their most defining characteristic (and no more)
- Repeat both name and characteristic often at first and at regular intervals later in your book.
After that introduction, slowly reveal more about each character over the course of the next chapters. We don't usually have infodumps about people we get to know in real life, either.
By rewriting the book so that the four main characters are not introduced all at once. There is a good reason that most books introduce characters one at a time or two at a time (in the form of a conversation). Readers need time to integrate each character and form a distinct memory of them.
This is no different from meeting people at a party. If you meet one person and talk with them for 20 minutes, there is a good chance you will remember their name and what you talked about at the end of the night. If you meet four people at once and they all talk, there is little chance that you will remember either their names or what they talked about.
This is just the basics of how we form memories. No writing technique is going to change that, except to space out the introductions so that the reader has a chance to form and retain memories of each one in turn.