Something about the statement of the task seems very odd to me. One way (I think the "literal" way) to interpret your description would yield a sequence like this:

0.  text {scope 4 {scope 2 {scope 1} scope 2} scope 4 {scope 3} scope 
+4} text

1.  text {scope 4 {scope 2 -change1- scope 2} scope 4 {scope 3} scope 
+4} text

2.  text {scope 4 - - - -c h a n g e 2- - - - scope 4 {scope 3} scope 
+4} text

3.  text {scope 4 - - - -c h a n g e 2- - - - scope 4 -change3- scope 
+4} text

4.  text - - - - - - - - - - - - -c h a n g e 4- - - - - - - - - - - -
+ - text
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So the question would be: why even bother with the initial nested levels, since later changes will obliterate them? If that's what is supposed to happen, it would make more sense to identify the outer-most bracketing, and apply only that single substitution. Perhaps you intended to describe something different?

(Update: I suppose that if there were interactions from one step to the next - e.g. if a substitution at step 1 either creates or eliminates a condition that affects what happens in a later stage - then it becomes a more complicated business, posing greater challenges for maintenance.)