So you have two options:
1. Change the shifters to Tiagra 4700 (and only Tiagra 4700 - not 4600 or any other model)
2. Change the rear derailleur to any 8,9 or 10 spd either road or MTB- doesn't matter. Only exception is Tiagra 4700 which is essentially the same as all the 11 spd derailleurs, just heavier.