Brief sex 4 days ago. today saw one painless large olive sized lymph node on left groin. no other symptoms or sores noticed. is this herpes? or what?

Not herpes: Almost certainly not herpes. There is no STD or other infection likely to cause lymph node enlargement in 4 days, especially without other symptoms like sores or irritation. The new lump in your groin -- whether really a lymph node or something else -- probably has nothing to do with the sexual event 4 days earlier. See a doctor if it persists, if any other symptoms, or you remain concerned.