They were of interest for various reasons, but ultimately Apocalypse isn't very picky (at least initially) about the horsemen that he picks because he expects them to die and be replaced by more powerful mutants over time.
Storm was very easy to manipulate into following him, and she provided some information about how to find other mutants to recruit, which benefitted him (she leads him to Caliban).
The next mutant he recruited was Psylocke, who because of working with Caliban was able to lead him to even more powerful mutants, which again was useful to him as a next step.
Psylocke: I know the kind of mutants you are looking for. And I know where to find them.
Next up is Angel, who has shown that he is capable of defeating other mutants in 1-on-1 combat in the cage match scene - it's why Psylocke leads Apocalypse to him in the first place. He also has the ability of flight (this ends up being useful in the final battle, even though he is eventually defeated).
Finally, Magneto is a particularly powerful mutant who is instrumental for Apocalypse's plan.
Apocalypse: Reach, as deep as you can. You'll find you have the power to move the very Earth itself. Everything they've built will fall! And from the ashes of their world, we'll build a better one!
So they do all have some use to him in the moment, but Apocalypse doesn't seem particularly picky about who they are - for instance, Storm is literally the first mutant that he encounters. He is also clearly not attached to them, as shown by his reaction to Angel's death:
Overall, Apocalypse's goal is dead simple: the strong will survive and the weak will die.
Apocalypse: Wipe clean this world. And we will lead those that survived.
And later on:
Apocalypse: To tell the strongest among you... Those with the greatest power. This earth will be yours.
In light of that philosophy, his recruitment method seems clear. He isn't picky about who his horsemen are initially because he knows the weaker ones will die off and be replaced by more and more powerful ones over time.