Access a control in a form loaded into a panel

I have a form loaded into a panel in my Main.cs ..

 

I'm trying to delete a file on a button click from Main.cs, based on the selected file from a combobox located in the other form.

 

I'm trying to access the filename from the combobox.text in File.Delete but having no luck. I've tried this.panel4.controls but it doesn't show up.

 

Any help would be appreciated.