Before we go any further, let's just establish what kind of suit we're talking about for this symbol. We don't mean this, we mean this (sorry to disappoint). Now onto our book.
When an Ace of Diamonds first arrives at Ed's door, he doesn't think much about the suit because he's focused on the much bigger questions of who sent it to him and why. But after a while, he starts considering the suits a little more carefully. Halfway through his journey, he shares:
I wonder which card will end up in my letterbox next. It's the spades that worry me most, I think. The Ace of Spades scares me—always has. I try not to think about it. I feel watched. (2.4.48)
Hmm… why would spades freak him out? Perhaps it has something to do with the fact that a spade is used for digging, and that might involve dead bodies in Ed's new line of work.
Ed's fears about Spades are buried (get it?) when he gets the Ace of Hearts and the challenges it presents are more difficult than he could have ever imagined because they're so personal. (Another way to think of this is that they are about matters of the heart. Get it?) Ed shares that "in moments of awakeness, I remember the diamonds, relive the clubs, and even smile about the spades. I worry about the hearts" (4.2.126). Looks like someone's afraid of digging deep when it comes to his personal life.
All this talk of suits makes us think about what Ed's required to do for each. Diamonds involves helping strangers—or looking for diamonds in the rough, if you will—while spades requires Ed to dig through books in the library. The Ace of Clubs involves our main man getting knocked out a couple times (or, you know, clubbed), and hearts is all about helping the people he loves. So it seems pretty obvious that the suits are symbolic of what Ed goes through emotionally with each card. The only question is: why does he think one is harder than the others? We're sure your teacher would love to get your thoughts in writing on that one.