Can 4 years of hair loss due to vitamin D deficiency be reversed once treated?

it matters how much hair you lost beacuse of the missing vitamin D. İf the ratio of the lost hair is on the high degrees, than even if you have a treatment the wole hair might not come back.

It is highly unlikely that Vitamin D intake at this point will restore your hair. However you should take a high quality supplement if you have a vitamin D deficiency. 

Hi, Vitamin D acts on the hair follicle to regulate gene expression to influence hair growth extending anagen phase and inhibiting catagen phase you can certainly take vitamin d to prevent further loss, you would need to get your blood drawn to see your level to determine how much vitamin d supplementation is needed typically 50,000 units once a week for 8 weeks then maintenance. Speak to your doctor first to see if you qualify. As if we could reverse hair loss only time will tell.

The deficiency of vitamin D has been known to cause hair loss. In case, you are still suffering from active hair loss, you should take it to prevent further hair loss along with other medical treatments. Now how many hair will come back because of the previous hair loss, only time will tell.

Damage from Vitamin D deficiency is often proportional to the deprived deficiency. Make sure your Vitamin D is normal, not too high or not too low. Keep it within normal limits (30-100). Potentially your hair should grow back within 6-8 months, all other factors being normal. Also you could try minoxidil with could be helpful as well. 

doesn't mention your age, but Propecia by mouth and some of the newer topical S like Viviscal and Nutrafol may also help.

Vitamin D clearly has a role in normal hair growth. But does supplementing vitamin D (in those with low levels) cause it to come back? No. I have never ever witnessed convincing evidence. It is absolutely true that low vitamin D is associated with some types of hair loss. This includes AGA (in some studies only) and alopecia areata. I do not believe that supplementing vitamin D alone will promote hair growth for the vast majority of people with the story you have just shared. Do I believe supplementing D is a good idea? Yes. It has other health benefits and may help various hair growth treatments work better. That too has yet to be proven.
I appreciate this is a tough concept and doesn't always make a great deal of sense at first. If low vitamin D has a role in hair growth and my levels are low... it would only make sense that supplementing would help! Not quite.
I often use a simple analogy. We all know that an engine is essential for a car to work. If you don't have an engine, you are not going to get too far. But consider the car that you see on an abandoned lot. It has problems with the engine sure. But there are problems with numerous aspects in the car. Replacing the engine is not going to allow you to drive the car off that lot. No matter how many engines you put in that car or how well engineered that engine is. Other things need to be addressed as well.
No one can argue that vitamin D is not important for hair. Babies who can't make vitamin D or who have problems with the Vitamin D receptor or signaling - have hair loss. So clearly it's important just like an engine in the car is important.
But giving back vitaminD to adults with hair loss or giving back a new engine to a car on an abandoned lot does not solve the main issue.
I always supplement vitamin D when low (and I would recommend fixing an engine on a car that does not work) but it is far to big of a jump to conclude that that alone is sufficient to grow hair (or get that car running again to drive off the lot).

Reversing hair loss from Vitamin Deficiency after 4 years may be a challenge. Of course, go on Vitamin D and start correcting the problem. If you also have genetic hair loss the problem increases.  Proper diagnosis with an expert in this field is critical