I would expect that level 4 only compares locally installed
certificates, however I get the same behaviour as with level 3, stunnel
expects a CA cert.
Here'e the relevant log when on level 4

Jul  6 23:46:31 mmm stunnel: LOG7[7870:140491349628672]: Starting
certificate verification: depth=0,
/C=qq/ST=qq/O=qqq/OU=rer/CN=redf/emailAddress=rfd
Jul  6 23:46:31 mmm stunnel: LOG4[7870:140491349628672]: CERT:
Verification error: unable to get local issuer certificate
Jul  6 23:46:31 mmm stunnel: LOG4[7870:140491349628672]: Certificate
check failed: depth=0, /C=qq/ST=qq/O=qqq/OU=rer/CN=redf/emailAddress=rfd
Jul  6 23:46:31 mmm stunnel: LOG7[7872:140080853112576]: SSL alert
(read): fatal: unknown CA

What am I missing in understanding verify's level 4 ?