I don't think that's a Phlox - the back of the flower bud clearly visible in the third pic resembles Dianthus (carnation) of some sort. No idea which one this might be, the flower form itself is not familiar to me, but they are quite sprawly plants if not given support.

Could it be a Dianthus as it grows to about 4ft?

4 feet does seem very tall for any Dianthus, I agree. But I'm still not convinced its a Phlox - let's hope someone else recognises it.

I too don't think it is a Phlox much more like a Dianthus certain species can grow to a good 3 foot.

I think it's saponaria--soapwort, Pdb. It grows wild along the coast and inland around here, South Cumbria, and is a very attractive plant. It can grow up to 4ft. high, so if you want to keep it, as I would, put some twigs or small canes around it before it grows too tall next year, and tie some twine around them to hold it up.
It isn't a Dianthus, because the leaves are wrong for that, and Phlox have leaves similar to these, but they grow right up the stem to where the flowers are. Look up Soapwort on the net, and I'm sure you'll find out that's what it is. They used to make soap out of it a century or more ago, hence the name.
Hope this helps. Annie