I have two editions of this work, the second I bought in order to give the examiners last summer something a little more presentable. Both are by Schott, the first bought for pennies at a music school fundraising sale about 30 years ago (probably the original 1943 UK imprint and now very much falling apart), and another purchased via my local sheet music emporium earlier this year, which appears to be a 1971 revised edition.
Both editions mark a '3' above the group of notes in question, as it is indeed a triplet lasting the second 16th note of the second 8th note beat as it were (the piece is in 4/8). The piece is not horrendously fast (1/8 note = ~100), and the triplet figure should be quite clearly articulated each time it occurs. Initially one can count mentally in 16th notes and perhaps play half to 2/3 of final tempo to make sure that one is not falling into the easy temptation of starting the triplet figure too early, if you see what I mean.
Interestingly the directions are not the same in each edition, the second edition using mostly Italian terms and the first being mostly in English. E.g. in bar two of the Praeludium the old edition is marked 'broad'
but the revised one has it as 'largamente'
. Similarly this particular fugue is marked Quiet
in the old book and Tranquillo
in the later one.
I have the Mustonen, Mauser and Richter recordings. I probably like the Mustonen version best. I selected Fugue 4 with the preceding Interludium to play for an ABRSM diploma examination last July, and am currently at the memorisation stage with his Second Sonata. Along with Prokofiev and Stravinsky, Hindemith is one of my favourite composers of the early 20th century. Good luck with your 'rediscovery'
 Closer inspection reveals the first was printed in England and the latter in Mainz.
 First two movements are not too bad in that regard, but the final Rondo is taking a while to sink in... too many notes/harmonies going every which way, I suppose that comes with the territory with Herr H