Total of four silicone breast implants, surgery in 1988/1989, explant and replace with new implants? Suggestions?

First, don't be afraid of "systemic illness" from the implants. That does not happen. Local issues do arise as with any device in the body. Implant stacking was briefly popular especially with polyurethane covered prostheses at the time of your surgery. If you are having no problems with them they can remain. An MRI will help define any internal issues. Explantation is an option should you choose.

Let me put your mind at ease.  There is no such thing as "breast implant illness".  That was a scare propagated by the media and, unfortunately, by unscrupulous attorneys. Since that time, numerous studies and scientific panels have demonstrated that women do not develop auto-immune/rheumatologic diseases from silicone implants.  There have been some  who have the older silicone implants who  have developed local (breast) symptoms of pain, swelling, inflammation, but this is not common and can be alleviated in most case by explantation.  The current recommendations are that removal of older silicone implants is not warranted unless symptoms are present..