I got sting bye wasp on my butt 4day ago.i red and swollen and pain?

Could be normal: It is typical for a wasp sting to become red and swollen and hurt. However if the pain,redness and swelling is becoming worse after four days rather than better you could be developing an infection and should be seen by a physician

Normal sting sx: It is not unusual to have redness and swelling after an insect sting or bite. Topical or oral Benadryl, Tylenol, (acetaminophen) and OTC cortisone cream should help your symptoms. If you are running a fever, if the red area is enlarging, or if you have increased pain or discharge at the site, see your physician to rule out an infection (cellulitis or abscess).

Injury: red & tender: With any injury you will have some level of redness, swelling, tenderness/pain, and or warmth/heat at the site of injury. This is the natural healing process and is called "inflammation". It is NOT natural is if there is too much inflammation (VERY red, painful, etc.) which might mean you have an infection. You should see your doctor to check for infection that might require some antibiotics.