I want to switch to 46/30 but don't know how to begin. The crankset is Shimano Tiagra FC-4700, 50x34T, 170mm

left side view of the crankset right side view of the crankset

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    The smallest chainrings you can get for your 110mm BCD crankset are 34 or 33 teeth. – Michael May 18 at 8:56

The crankset (110 BCD - Bolt Circle Diameter) is unlikely to accept a 30T chainring. It definitely isn't supported by Shimano. You need a new crank.

Look at subcompact cranksets like, e.g., FSA Tempo Adventure (10/11 speed). Note that this specific one also uses a different bottom bracket.

  • Thank you. I didn't know that there's a BCD and what it was. But after your answer I googled it and found this article: corbah.com/blogs/bike-talk/… . Maybe someone else will find it useful: contains a table for different BCDs. – Zloj May 18 at 10:02

Shimano offers the FC-RX600-10 in their gravel bike line which comes with 46/30 chainrings. The chainline is a little outboard of the road chainline (by 3.5 mm). You may be able to adjust the stops on your front derailleur to fit or you may need a new gravel one. I also don't know if it fits your bottom bracket.

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