Welcome Harley The Hi-Vi tweeters you linked are great replacements for those in the AR-3a but won't work well in the 4x. The tweeter crossover in the 4x is at 2,000 Hz IIRC. The Hi-Vi's frequency response is 2,500 - 20,000. Your best bet is to use original AR-4x tweeters. Some people (myself included) have reported some success using phenolic ring tweeters like these https://www.parts-express.com/grs-prt-8-phenolic-ring-tweeter-replacement-for-ar-4x-cts-marantz-and-more-8-ohm--270-252