Joe --

no ideas on the sheet music, but "Hob. VIIa: 4" is a reference to the number of the piece in the catalogue of Haydn's works done by Anthony van Hoboken (1887-1983): "J. Haydn,
Thematisch-bibliographisches Werkverzeichnis." The roman numeral is the type of composition, the other number is the number assigned to the particular piece. In the more-than-you-probably-wanted-to-know department, following is a list of the musical types and corresponding Hoboken numbers.

michael

# Symphonies H I 1-108
# Overtures H Ia 1-16
# Divertimenti in 4 and more Parts H II 1-47
# String Quartets H III 1-83b
# Divertimenti in 3 Parts H IV 1-11
# String Trios H V 1-21
# Various Duos H VI 1-6
# Concerti for Various Instruments H VII
# Marches H VIII 1-7
# Dances H IX 1-29
# Various Works for Baryton H X 1-12
# Trios for Baryton, Violin or Viola, Cello 1-126
# Duos with Baryton H XII 1-25
# Divertimenti with Piano H XIV 1-13
# Trios with Piano, Violin or Flute, Cello H XV 1-40
# Piano Duos H XVa
# Piano Sonatas H XVI 1-52
# Piano Pieces H XVII 1-12
# Piano 4 Hands H XVIIa 1-2
# Piano Concerti H XVIII 1-11
# Pieces for Mechanical Clock (Flötenuhr) H XIX 1-32
# Instrumental Works about The Seven Last Words H XX
# Choral Version of The Seven Last Words H XX/2
# Oratorios H XXI 1-3
# Masses H XXII 1-14
# Other Sacred Works H XXIII
# Cantatas and Arias with Orchestra H XXIV
# 2, 3, and 4 Part Songs H XXV
# Songs and Cantatas with Piano H XXVI
# Canons H XXVII Sacred 1-10 Secular 1-47
# Operas H XXVIII 1-13
# Marionette Operas H XXIX
# Incidental Music H XXX
# Arrangements of (273) Scottish and (60) Welsh Folksongs H XXXI