i have been using .380, 9mm , .357 and .38 bullets inter changeably for a while now.
with OK results, all i have been doing , is making plinking loads with very light charges, so far ive no problems.
i feel what i have been doing , is ok and not unsafe.
what works best is using .380 bullets ( 90 grains ) in .38 slp or .357 mag cases.
using the smaller 9mm ( .355 ) bullets do work but they are not very accurate and seem to get my guns dirty fast (blow by maybe ? )
now you are using that 4 inch barrel, so you should not see any problems with blow by.
you should be able to load those 9mm ( .355 ) bullets in the .38 spl cases, using a light powder charge and shoot them just fine.
just my 2 cents.