How do I improve specific gravity in a 4 year old battery
In most cases, take it on a trip to the recycling center, or trade it in to be recycled when you buy the replacement. Lead acid batteries are a consumable item in solar service, and more so if the solar service is poorly managed such that they are not brought to full charge, or are over-discharged on a regular basis. They are a finicky and fickle part of the system, but less costly than batteries that are more tolerant of imperfect operating conditions.

You can charge the heck out of it, but it's probably past saving that way. You may be thinking "I can just add acid" but that does not actually work since the problem is not lack of acid, but where that acid has gone, chemically. Add sufficient distilled water, only, but I very much doubt you'll bring it back from the grave.