The easiest way for you to do this is to use the Multiple argument. This will automatically add an extra field to your set, so you will always be able to add more fields to your slideshow.
This is also a lot neater codewise.
type = file
label = Upload Main Image ( 940px by 280px )
mediaLibrary = true
multiple = true
multipleButton = true
"Multiple" will allow you to add more fields so that when you upload 4 and save - it will automatically show your 4 fields and 1 empty one when you next load the page.
"MultipleButton" will allow you to manually add more fields, if you wanted to add more than one at a time.