Me is the VB.NET equivalent of C#'s this. So in C# you'd write tinstaafl's suggestion as:
for (int i = 0; i < 4; i++)
this.Controls("pictureBox" + i).Visible = true;
Though this depends on each PictureBox being default named as if you dropped them onto the form and didn't change the name. It also assumes that all four PictureBox controls exist directly on whatever control/form you're referencing by this.
All in all, pretty big assumptions given how vague your question is. I'm also not sure that the loop does what you want (actually, I'm pretty sure it doesn't). We need more information as to the exact behavior you're looking for before any specific suggestions can be made.