The #4s are main bearing cap side bolts.
They lie above the oil pan as shown.
I would replace the bolt as well as sealing it. Some bearing bolts take a permanent strain when torqued to specs and cannot be reused.
BTW, was it this forum that was discussing left-handed threads the other day? FWIW, items #3 above are left-handed threaded.
Sealing the leak is not difficult if the bolts can be reached. Any good sealant should work.