I have never stored NBF at 4C since the cold temperatures will cause the
paraformaldehyde to come out of solution, a white flocculant looking
ppt floating on the bottom of containers.  This ppt does not go back into
solution, and your NBF may be off base when this happens, a changed conc??
I have cooled NBF to 4C for a short time, using ice, and 
cold fixed in a refrig overnight. I believe that
manufacturers of NBF tell you, in spec sheets, to store NBF at RT, not cold,
and I have been advised to put in stocks ahead of cold winter shipping 
conditions, particularly to Montana, where temps in trucks can be extremely
cold, sometimes freezing.  Hopefully some of our NBF vendors will comment
on this.    

Paraformaldehyde, made up in a buffer, is another story, is stored in a
refrig to maintain it.  We prefer to make up 4% paraformaldehyde in Dulbeccos
PBS, store at 4C.  Have fixed frozens and cell cultures with cold PFA.  The PFA
is checked for any ppts in bottom of container, if found, it is discarded
and fresh is made.  

Hope this helps.
Gayle Callis