I first looked for other instances of "fourth-pair window" online, but drew a complete blank.
I then Googled the poem, but there are less than ten sources for it to be found among the resulting hits (at least at my time/space coordinates); and of those few, most seem to be duplicates (specifically, of an OCR'd version of the poem apparently taken from Littell's Living Age/Volume 127/Issue 1644).
Next, I took down from my bookshelf a couple of old poetry anthologies dating from 1932 and 1936. Though they both contained some poems by William Ernest Henley, this one was not among them.
For good measure, I also looked up pair in my copy of Chambers Concise Scots Dictionary (since the location of the house in question is probably Edinburgh, the only town of substantial size that overlooks the Firth of Forth), but found no connotation in Scots that had any relevance to windows. (Edinburgh is also built on a series of steep hills, which would account for "Like frozen waves the roofs go rolling down / The valley steeps".)
The reason for my quest for an original paper-based version of the poem -- or a photographic scan of one -- was to enable me to eliminate the possibility that "fourth-pair window" was an OCR-generated error; a reasonable suspicion, given that no other instance of the term can be found in Google's compendious archives.
(Nor, for that matter, did I encounter any Google hits for "first-pair window", "second-pair window" or "third-pair window".)
All this circumstantial evidence relating to the source of the text causes me to suspect that "fourth-pair window" is actually an OCR mistranscription of "fourth-floor window" **.
Within the poem itself, there are further grounds for supposing that Henley's wording was actually "fourth-floor window". Specifically, the narrator is high up enough to be able to look across chimneys, roof slates and roof ridges, and to glimpse the estuary of the Firth of Forth. In 1875, the technology for constructing buildings around steel frames had not yet been developed, so five storeys was then about as high as you could go.
An answer will be at least one step closer once a genuine printed text of the poem is available; it will dispose of the OCR issue, and if my surmise about a transcription error is correct, it will actually answer your question as well.
If it turns out that the original text really does say "fourth-pair window", then your quest for an answer will have to continue in a different direction.
** This would be known in the US as a fifth-floor window, owing to the different way the storeys of a house are numbered there; in Britain, the customary progression is ground floor, first floor, second floor etc., whereas in the USA it is ground floor (or first floor), second floor, third floor etc.