Learn English – “Paintings on walls and ceilings” and “painting of portraits, landscapes”

The etymology of diptych and its extensions triptych and polyptych includes the concept of the folding of the images.

Mid 19th century: from late Latin polyptycha (neuter plural) 'registers', from Greek poluptukhos 'having many folds', from polu- 'many' + ptukhē 'fold'.

ODO

As Chasly has commented, the concept of folding a single panel is rather self-defeating, so monoptych, which would be etymologically correct, is not found.

Generally, as the linked paintings are painted on board, the term panel is used.

At the U.S. magazines where I've worked, the terminology has always been front cover, inside front, inside back, and back cover.

In the past (and perhaps in some places even today), however, U.S. publishers may have assigned numbers to each of the four surfaces in question, in much the same way as (according to the OP) Russian magazine publishing does. S. Roland Hall, The Advertising Handbook: A Reference Work Covering the Principles and Practice of Advertising (New York: 1921) uses this nomenclature:

A few magazines sell part of the space on their front cover. The front cover is undoubtedly the most valuable position in magazine-advertising. The cover pages should ordinarily rank in this order, according to the various tests that have been made.

First cover

Fourth cover

Second cover

Third cover

The second cover has an advantage over the third only because the reader begins at the front of the book, and there is always some chance that he may discontinue his thumbing over of the pages before reaching the last page. The fourth cover gains second place in value largely because a magazine is left much of the time in such a position that the fourth cover is in full view of the reader.

This same handbook states that all of the other pages in a magazine are called "white pages"—so the interior page facing the inside front cover is termed "the first inside white page" and the interior page facing the inside back cover is termed "the last inside white page."

One contemporary source ("Must-Haves for Creatives" on the MBASkool website) indicates that the "first cover," "second cover," "third cover," "fourth cover" wording remains current, but most of relevant matches from a Google search for "first cover" + "second cover" + "third cover" + "fourth cover" are from the early 1900s. If the numbered terminology does persist at some publishing houses, it may be more common on the ad sales side than on the editorial side. In any event, I have never heard anyone in publishing use it.