i was wondering about any advice you could give regarding playing 4ths on the bass and extended 4ths based lines, particularly in thumb position. how do you finger them etc. im playing 2 consecutive 4ths in thumb position using fingers 2 and 3, but how about those exercises in the richter book that have groups of 3 4ths moving around the keys?

Using 2,3 fingering for 4ths works well. For example : Bb on the G string 3rd finger, F on the D string 2nd finger. Add the C on the A string with the 1st finger. Moving this "bridge" around can work, but is very challenging.