"...four others, one of whom responded." Is “whom” correct here? Can I use "who" instead?

Whilst there are some posts to a question here that deal with some aspects of the usage of whom, none of them address the issue arising from the Original Poster's question here.

Many posters on ELU (this site) write about the death of whom in natural English. The death of whom, however has been over-exaggerated. Many of the writers foretelling the demise of whom advise using who in all instances or reformulating the sentences in which it occurs. However, these writers advising the avoidance of whom are meting out the same prescriptivist advice that right-minded descriptive grammarians abhor. This advice in fact can be positively damaging because in the modern grammar we often actually have to use whom instead of who to make our sentences grammatical.

The most important of these situations is when who/m is the object of a preposition in a relative clause construction. In such situations, who is often quite straightforwardly unusable. The following, using who, are all ungrammatical:

• *There were twenty students present, several of who had been seriously injured.
• *There was one woman about who I had heard nothing at all.
• *Those people, not one of who has ever done anything to harm us, deserve better.

Contrastingly, the following are all fine:

• There were twenty students present, several of whom had been seriously injured.
• There was one woman about whom I had heard nothing at all.
• Those people, not one of whom has ever done anything to harm us, deserve better.

Now, some writers might argue that these sentences should be reformulated so as to avoid the use of whom. However, this is complete piffle. To do so would be merely to pander to the personal predilections and prejudices of certain writers. It would have nothing to do with either grammar or good style. The only reason to suggest reformulating these sentences would only be so that the personal predilections of such writers held true.

Here is why these sentences shouldn't be reformulated. In each case the consideration is one of information packaging. Information packaging is merely about how we present new and given information to achieve the emphasis we want and to make our sentences easier to process.

In the first example, the formulation of the sentence requires contrastive stress on whom making the sentence more emphatic - this would be lost in a reformulation. This stress ties the second clause in more tightly to the first. In addition, the sentence construction builds up the readers anticipation as the sentences progresses, rendering the focus of the sentence - the injuries - more potent.

In the second example, if reformulated into two sentences, the first would become quite redundant:

• There was a woman. I knew nothing at all about her.

The second sentence in the reformulated example, also lacks the focus of the original. The original puts the emphatic nothing at all in the focal position right at the end of the sentence. The reformulation puts about her at the end. Hardly the same effect.

In the third example, I cannot really see how a viable reformulation would work. Whichever way it was done, it would lose its emphasis:

• Those people deserve better. Not one of them has done anything to harm us.

As can be seen the reformulation above puts the emphasis on harming us as opposed to deserve better.

The Original Poster's Question

The question of which of the two formulations is best in this instance is mute. It depends entirely on the context of the sentences and the objectives of the writer. However, in terms of the grammar I sent emails to four others, one of whom responded is clearly grammatically correct. I sent emails to four others, one of who responded is clearly ungrammatical.