My DD will be 4 in June, she is an only (not sure this makes any difference). She told me the other day that she is lonely at preschool and is being left out, the thing that doesn't help is that she is SO quiet.
I'm now starting to worry about her being excluded, ignored by teachers, left out of invites (all the stuff that happened to me when I was a quiet child).
The first step is to speak to her teachers to make sure she really is being left out. Many children report this but in fact, what's happened is that there was maybe one instance of lonliness which becomes inflated in their minds...being 4 they only feel the intensity of the moment and don't have the ability to think "Well never mind, it was just today" and so catastrophize it.
The second step will be to enroll DD in something extra carricular like Rainbows...and to invite some other children to your home.
Up her social experiences and encourage her to speak out.