As a preschool teacher, I would not worry about reversals or writing form.
The development of physical act of actual writing (small muscle control) and the process of writing (sound-letter identification and visual spacial cues) are different.
Many early readers/writers reverse and it is developmentally OK and not unusual to age 7.
Students, especially earlier or independent learners, often will write completely backwards, mixed formats, and reversals. The asynchronous development is not uncommon. I would keep an eye on it, but not go running for an eval in that area. Keep an eye on his eye development. Sometimes eyes dont work well together and that results in writing concerns and/or difficulty. Or fine motor control. Most specific assessments for 'writing' will not identify anything at age 4/5 because writing at all in letters, words would place him out of a 'delay' range.
As for correcting him, I wouldn't. It will come in time and phonetic spelling, unusual spacing/capitalization are part of the writing process developmentally. If he is writing and spelling-- that is ahead of the developmental range for a 4 yr old but follows the developmental curve for writing development.
If you correct him, he may develop a hesitation to write for fear of mistakes and/or correction rather than freely writing (as it sounds like he is doing now).
Does he have a dominant hand? Many kiddos with mixed dominance will write upside down, reverse for a bit longer due to the way the brain processes information.
One of my girls wrote in preschool all sorts of ways backwards, reverse, etc. She did not pick a hand until 1st grade. She even reversed numbers/letters to 2nd grade. Now in 4th she does not. Slowly she self corrected, she is very visual spacial and excels in puzzles and visual processing. She , too, still prefers to do math in her head since the act of 'organizing' in writing is much harder than just figuring it out.
