Help! Which sentence is correct?
I want to say: I found a problem when I filled out the form.
But I want to express it differently

Which one is correct and nature? (and what are these grammars called)
1. I found a problem filling out the form
2. I found a problem, filling out the form
3. I found a problem while filling out the form
4. I found a problem when filling out the form

Question

Help! Which sentence is correct?
I want to say: I found a problem when I filled out the form.
But I want to express it differently

Which one is correct and nature? (and what are these grammars called)
1. I found a problem filling out the form
2. I found a problem, filling out the form
3. I found a problem while filling out the form 
4. I found a problem when filling out the form

Answer

Editing my answer to say that Robert has the correct tenses. It would also be correct to say: "When I was filling out the form, I found a problem," or "While filling out the form, I found a problem." Hope that is helpful.

Answer

I find a problem whenever I fill out the form.  Present tense find/fill denotes habitual action, more than once
I found a problem when I filled out the form. Past tenses denote past action, only once