4yr old writes in mirror image. All letters are backwards & right to left.  for example, he wrote "red" as "der" and all letters faced left not right?

Writing letters back. In a four year old writing letters and numbers backward could be normal. However by the age of 5-6 years eriting letters and numbers backward is definitely abnormal and may be a sign of add.
Decoding shapes. The maturing brain acquires the ability to see and decode shapes &orientation over time and some kids get it earlier than others. By 6 this should be a rare occurance & schools often screen for the larger issue of dyslexia @ that age.You can access screening thru the "child find" program operated by most schools if concerned. Add kids can have dyslexia but i don't consider it a sign of add.
Normal. This is not uncommon in a 4 year old. This is called mirror writing. If it persists beyond 5 to 6 years i would seek help.You can help him by reminding to write from right to left by placing a marker on the left upper side of the writing area with a marker or sticker.