The considerations here are much the same as those I discussed in [a previous answer][aa]. I have attempted to develop those ideas and tailor it to the passage in question.
Yes, it is.
In Greek, the subject of a clause can generally be identified as the substantive in the nominative case. However, in Greek (as in English), some sentences have two nominatives; this is known as a predicate nominative construction. It occurs when the verbal idea is "equative" rather than active. In 1 John 4:8b, we have:
We can apply these criteria to 1 John 4:8: ὁ θεὸς (=God) has the article and is therefore the subject.2
The implications of distinguishing S and PN are similar in Greek and English. Most often, this frames a subset proposition. S is the narrower category (the hyponym); PN is the broader category that subsumes it (the superordinate). In this case, God is a specific individual within the broader category of love.23
2. [Augustine's homily][ah] notwithstanding (see note 2365). I can't speak to the appropriateness of the Latin formulation.
3. Of course, such propositions do not normally mix individuals and abstract concepts. This is intentionally, I think, jarring and characteristically Johannine (cf. 1:5, "God is light" and 4:24 "God is spirit"). Fortunately, we are not left on our own to operationalize this: "And this is how God's love was revealed among us: that God sent his only son into the world..." (4:9).
