The subwoofer section in particular is made of one Dynaaudio 30w100 coupled with a 30w54. Surround kits abound for these irreplaceable drivers, but rubbing or burnt out coils are trouble. The rest of the drivers would basically be upgrades from Vandersteen at this point. The tweeters might be problematic as the Vandersteen 4a is 5-way design with two tweeters compared to the 4-way Vandersteen 5s with one tweeter.
I've actually had to send a 4a into repair at Vandersteen and basically they have an inventory of drivers to match what is in your speaker; they have the actual measurements from your pair and will match as best they can. The expensive Dynaaudio subs he won't have; I'd be surprised if he was out of the tweeters, but I'm sure a better one could be done in the speaker by someone who knows speakers.
The problem with replacement drivers is often Richard won't have an exact match. With the 4a's if you modify them to tri-amp then you can overcome any matching issues via level controls and amp level controls. With the 4's design the level of the super tweeter and the the 8 inch mid-woofer are locked together if you only bi-amp.
Do not buy used Vandersteen 4's if you are trying to have the best example of these speakers. The 4a has all new drivers and was almost a $2000 upgrade back in the day; it would cost a fortune now to take a 4 to a 4a. 4's are fine cheap and kept as they are. The 4a with the right setup is a mighty speaker, but the cabinetry at best matches a stock model 5 for sound. The strength of the 4a is those Dynaudio 12" woofers which need real Van den hul Magnum and the right amplifier to rival or exceed the current models. At the time 4a's were 5% of Richard's sales, but 95% of his calls; the key is the subwoofer section and ancillaries. Richard with the model 5's built in amplification eliminated that support issue lol.
