I have a Shimano Tiagra Triple 30/39/50 and I want to find out what is compatible in terms of smaller chainrings for touring. Ideally, I’d like to be able to go down to a 26 or even a 24 for the smallest. Thinking in term of 26/36/46 or 48. This is a 4 bolt crank and it seems there’s little to no compatibility with other sets.
Unfortunately the asymmetric four bolt pattern that Shimano switched to for these cranks is a new patttern and so far there are no small inner rings available. The best advice I can give is to switch to the older five bolt style crank, for which many small rings are available. I use 26-39-50 on the previous generation of Tiagra with no trouble.
Do you know what BCD is used for the smallest ring? 4 bolt holes could be 65mm or some other size.
Assuming your crankset is a FC-4703 then the specs are at https://bike.shimano.com/en-EU/product/component/tiagra-4700/FC-4703.html and suggest its 74mm BCD.
Sheldon says that a 74mm BCD will take a 24 tooth chainring, or 26 and 28 options as well. He also suggests a "deflector" to help the chain make the big jump down from the 39 without missing, to prevent damage to the frame.