Buy whichever derailleur the cheapest (4700, 5800, R7000 etc.).
(4700 have the same cable pull as 11 speed mean you can fit 105 GX derailleur on Tiagra 4700 shifter).
So 4700 STI (10 speed) will work with 11 speed rear mech on a 10 speed cassette?
Yup, 4700 have the same cable pull as the rest of the 11 speed groupset.
(I ran an Shimano MTB 11 speed derailleur converted with Shiftmate to 11 speed road cable pull on a Tiagra RS405 10 speed shifter).
@Valente started
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Buy whichever derailleur the cheapest (4700, 5800, R7000 etc.).
(4700 have the same cable pull as 11 speed mean you can fit 105 GX derailleur on Tiagra 4700 shifter).