Condensed answer: The components part of the Tiagra 4600 series are not compatible with those of the Tiagra 4700 series even though both groupsets are designed for 10-speed road bikes.
The sources of the incompatibility are the unique cable pull and shift ratios of the Tiagra 4700 units.
Indexed shifters have their movement split into separate audible clicks. Each click indicates a shift and moves or releases a certain amount of gear cable. That cable movement controls the derailleur and triggers a shift.
The amount of cable that’s pulled or released by the shifter is known as cable pull. The value of the cable pull is dependent on the number of speeds, the bike type (road or MTB), and the manufacturer of the component.
Rear Shift Ratio
The rear shift ratio indicates how much the derailleur moves per 1mm of cable pull. For example, if the rear shit ratio is 1.7, the derailleur would move 1.7mm for every millimeter pulled or released by the shifter.
The cable pull + rear shift ratio combination results in predictable movement of the rear derailleur and is behind the success of modern index shifting.
However, since both the shift ratio and the cable pull are speed and bike-type-specific, incompatibilities occur.
The Problems With Combining Tiagra 4600 and Tiagra 4700
Tiagra is a 10-speed road group set from Shimano. Thus, one can easily conclude that the two series come with interchangeable components, but this isn’t the case due to the dissimilar cable pulls and shift ratios.
The rear derailleurs from the Tiagra 4600 series have the rear shift ratio of Shimano’s 10-speed road models, namely 1.7mm.
Meanwhile, the rear derailleurs of Tiagra 4700 series have the rear shift ratio of Shimano’s 11-speed road models, namely 1.4mm.
Since the derailleurs of both series have dissimilar rear shift ratios, they are not interchangeable.
If you put a 4600 derailleur in a 4700 system, for example, it will not move to the needed location and will create a poor shifting experience.
The cable pull of the shifters doesn’t match either. Tiagra 4600 rear shifters have a 2.3mm cable pull that matches that of other 10-speed road shifters.
The cable pull of Tiagra 4700 rear shifters is 2.8mm and is therefore unique to the series. The Tiagra 4700 rear shifters have a unique cable pull because they have to move a derailleur with an 11-speed rear shift ratio across a 10-speed cassette.
If the Tiagra 4700 rear shifters were to use a 10-speed cable pull (2.3mm), they will “undershift”. If they use the official 11-speed cable pull (2.7mm), they will “overshift”.
FAQ: What about the front?
The cable pull of the front shifters and the shift ratio of the front derailleurs doesn’t match either.
The 4700 front derailleurs have a longer lever arm and thus demand a longer cable pull from the front shifter.
Consequently, it’s not recommended to mix 4600 and 4700 front mech and shifters either. Shimano’s compatibility chart confirms the incompatability.