#4 has been around a very long time, much longer that catalyzed urethane clearcoats. The whole formula was balanced around the paints that were put on cars “way back when.”
Which is not to say that it can’t work on new paint, only that it’s not optimized for it. And since new clear coats are very unforgiving when it comes to swirls a non-optimal product is less likely to give good results.
The abrasives in #4are very large compared to Meg’s newer formulas. You can actually feel them. They have a large initial cut and break down to a finer and finer cut. The idea is to remove a lot of material quickly and then remove marring left by the first deep cuts. If they break down at the wrong rate for a given finish they won’t be able to remove the marring and will just leave swirls, hazing or scouring.
A DA works the product very differently than a rotary and can leave scouring with #4. A modern catalyzed clear may need a longer work time or smaller initial cut depth or whatever.
For whatever combination of technical reasons, they found that the formula of #4 wasn’t keeping up with the needs of today’s finishes and paint shops so they created new products to meet the demand. They keep the old formulas around as long as customers keep buying them. Presumably, there are shops out there that have their processes dialed in with the older stuff and are happy to continue doing what works for them.
PC.