I also find it difficult to compare an entire run of a graphic story with single volumes from series with which I am familiar and also a single volume from a series that I don't know. Even if you try to restrict your judgement to the material listed on the ballot paper, the fact is that your enjoyment and understanding of the volume in question is massively informed by your knowledge of how and if it fits into the wider narrative of the series as a whole (or even the author's work as a whole).
My votes in this category therefore leave me feeling a little uneasy. But I will cast them none the less as follows:
1) Digger by Ursula Vernon
2) The Unwritten (Volume 4): Leviathan, by Mike Carey, illustrated by Peter Gross
3) Fables (Volume 15): Rose Red, by Bill Willingham and Mark Buckingham
4) Locke & Key (Volume 4): Keys to the Kingdom, by Joe Hill, illustrated by Gabriel Rodriguez
[no vote] Schlock Mercenary: Force Multiplication, by Howard Tayler
See also: Best Novel | Best Novella | Best Novelette | Best Short Story | Best Related Work | Best Dramatic Presentation (Long Form) | Best Dramatic Presentation (Short Form) | Best Professional Artist | Best Fan Writer | Best Fan Artist | Best Fancast | The John W. Campbell Award (Not A Hugo)