slide values over if right textfield is missing value

I have 4 textfields - a string, supplied by the user - is auto-split into each of the four fields from another textfield - so, e.g. "This is a test" - each of the four words gets put into individual textfields.

What I needs to do is make certain, all textfields, starting from the right - always get a value - so, if the user only supplies - "This is test" - textfield 4 = "test", 3 = "is", 2 = "This" - and 1 is null/blank...?  Otherwise - just leave as is, if all 4 textfields has a value

Need to do this in C#