Welcome to the forum.
While this makes sense from a programing perspective that “Loops” code, in 3d printer gcode the program is only run once, top to bottom. It doesn’t have the ability to use canned cycles that loop back. If you go the conditional statement route you will need to insert it on every layer.
I think I would try inserting it at the bottom of the “Layer change” gcode.
Here is another “simpler” option.
The slicer only has provisions to have a different initial layer temp.
How I suggest you approach this is to change the “Initial” and “Other” layer temps in the filament settings to what you want the first 3 layers to be.
Then simply use the slicer to insert a "M104 S(Whatever temp you want for the subsequent layers) at the beginning of the 4th layers.
