The Greek wording, from the undisputed text of the Received Text (Stephens, Beza, Elzevir and Scrivener all identical) is :
πνευμα ο θεος [John 4:24]
Young's Literal, The KJV, J N Darby and Tyndale (all four from the Received Greek Text) and The Wycliffe and Douay-Rheims (both from Jerome's Latin Vulgate) all give :
God (is) a Spirit
in which the copular verb (to be) is added for the sake of English idiom. The KJV and JND have italics to show that 'is' is not in the original Greek.
Green's Literal, unusually, has :
A Spirit God [is]
in the interlinear body of his text, but in the margin he reverts to the same as the above others and has :
God [is] a spirit.
Jesus' words are 'Spirit, the God' which, since the article is used and therefore is expressing a concept (the concept being identified by the Greek article) could be expressed in English as 'Deity is Spirit'.
I would suggest that the concept which Jesus expresses is a matter of Divine nature. He is saying, I would suggest, that the nature of deity is a spiritual nature.
Given all of the above information, I do not think there is any ambiguity or that there is any difficulty of understanding this statement.
The only issue in this translation is the question of giving an idiomatic representation in English to the concept which Jesus spoke in the Greek language, which, once one examines it, is a profound statement indeed.
All of the translations, in the question and above, endeavour to be helpful by adding the copular verb, which is quite a reasonable thing to do . . .
. . . except the NKJV which seeks to be literal (God is Spirit) but that is conceptually incorrect as there are other spirits (angels) who are not God. So that is actually an unsatisfactory translation from a logical point of view, given that the Greek identifying article is used by Jesus in front of θεος, Theos.
The fact that English (unlike Hebrew and Greek) has an 'indefinite' article causes a lot of conceptual problems with translation. It would be better if English did not have such a thing.
What Jesus expresses can be often seen in Hebrew when an equivalence is stated without a copular verb. The statement 'Spirit the Deity', although not grammatical English, does convey an equivalence and would be a literal translation, as the English word 'Deity' does not express Person (in the same was as 'Father' or 'Son') but does express divine nature.