It is said here that EPSG:4326 is not a projection.
In my mind, a projection is a transformation of the surface of the Earth producing a flat image, notably a bitmap.
We know that WGS84 (World Geodesic System '84) is a model to reference the Earth surface. It's an angular coordinate system and not a projection.
When I request an image from a WMS server and I use EPSG:4326, I receive a bitmap image. Hence, in my mind, EPSG:4326 is both a projection and a coordinate system used to locate the image (its dots). The projection is
Mercator Equidistant Cylindrical and the coordinates are WSG84.
JOSM is said to be unable to reproject a bitmap, that is, to transform a bitmap image from one projection to another. However, if I choose EPSG:3857 as the projection JOSM uses to display the map "at the screen" (in a window), JOSM is quite able to display 4326 from the server (but not other projections). Hence, the image must be "the same", more exactly "not to be reprojected". This seems incoherent with what you have just answered (4326 and 3857 displaying differently). So what's going on there?
Have I got that right and is EPSG:4326 a projection or not? (in bold on iant's request)
BTW, I read this, seeming to say that an EPSG is both projection and coordinates:
"Les codes EPSG constituent une liste des systèmes de coordonnées géoéréférencées de projection"