Compatibility between 4700 Tiagra 10 speed shifters and GRX RX400 10 speed derailleur I want to build myself a drop handle bar touring setup bike.
GRX RX400 seems like a good budget option. Bikeradar mentioned this groupset to be on same level as Tiagra. Gearing range on this groupset is as per my need. 11-36 in back and 46-30 in front.
Reason why I want to use Tiagra shifters and not GRX shifters (ST-RX400) is because GRX shifters are for hydraulic disc brakes only&gt;. I have never installed
or operated hydraulic disc brakes on STI shifters and I'm not sure how complicated the process of resizing the hose will be, so want to stick with ST-4700 rim brake compatible STI shifters.
Looking at Shimano's Compatibility Document one can see that indeed, Tiagra ST-4700 brifters are compatible with their rear derailleur model RD-RX400.
Can't blame you for wanting to stick with rim brakes.  Although hydraulic disc systems are not as complicated or difficult to work with as it may seem, there are some negative aspects that (IMHO) make a quality rim break quite competitive with disc brakes.  From a cost stand point--both initial and continuing maintenance-- rim brakes have a lock on value.  The relatively minor edge on performance disc brakes have is a fairly costly one.