If I understand correctly, you want to remap “Take a screenshot and send it to Preview” to ⇧⌘4 (from originally ⇧F4).
Basically it goes like this:
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Replace the hotkey in the macro header with your new one (as @hello already had said)
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Adjust the modifier-key conditions in the macro accordingly:
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The modifier for ‘sending to Preview’ is ⇧. This is OK, since ⇧ is also in your new hotkey.
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The ⌘ modifier however is used for switching fullscreen/selection mode, aka ‘Interactive toggle’. So, you would have to replace that condition with a different one that is not part of your new hotkey. A quick way would be to use ⌃ (Ctrl) instead, which is not used at all in the original macro. (It's defined in the action titled “Modifier for Interactive Toggle”.)
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Now adjust the hotkey triggers in the macro header to the changed modifier condition(s): In this case you would have to replace ⌘F4 with ⌃F4, and ⇧⌘F4 with ⌃⇧F4.
For consistency you could adapt the text of the dialog window (where it says “⌘” for the Interactive toggle) to the new modifier.