easy with regex
find </p>\s*<p class ="calibre4">([a-z])
replace \1
NB the replace line has a space in front of \1
works with v4.2 regex, I have not upgraded to 0.5.1 yet.
pretty sure there are other threads on how to do this, a varienat for example is to test end of previous line for not fullstop not quote not questionmark