Neither "4% paraformaldehyde" nor "4% formaldehyde" accurately describes the solution, which is actually 4% methanediol (i.e., formaldehyde hydrate).

"4% paraformaldehyde" is a legitimate description of how the solution was made.  It suggests that the solution was fresh and that very little formic acid or methanol had had  time to form.

"10% formalin" is generally understood to mean: "I added 10 ml of what my supplier called 40% formaldehyde to 90 ml of water.  I have no idea how pure the stock solution was."

Unfortunately "4% formaldehyde" is ambiguous because it does not tell how the solution was made. 

Allen A. Smith, Ph.D.
Barry University
School of Graduate Medical Sciences
    Podiatric Medicine and Surgery
Miami Shores, Florida  33161-6695