I had both a Mechanical & Fingered Extension on the same bass in the early 70s. I was in a car accident with the Bass and when the Bass was repaired we went for a handmade fingered Extension for a few reasons. First off, the Mechanical extension was damaged in the accident. Second, you could not use vibrato. Third, the keys were some times confusing and noisy if something came up fast in the music or in your head.
The Mechanical Ext. cuts down on intonation errors but the fingered method cuts down on 'keying' the wrong note.
For Jazz or commercial work, I favored the Fingered Ext.. For Classical, the Keyed method is more accurate for fixed Bass parts.
I have an Extra large old Bass in restoration now that will get a Fingered extension when it's completed. I will use it mainly in orchestra but I will have more flexibility for expression and other musical styles as well.........
I did own one 5-string Bass for a short time in the mid 70s, but I convereted it to a 4-string and sold it. 5 was not my thing then and still isn't now.
For the few lower notes I might play on occasion, I would prefer a normal 4-string with or without a fingered Extension.
We can always tune down if necessary and write in the fingerings up to the 3rd pos. b-4 moving to the A-string. In my last Orchestra Concert locally doing Movie themes we playey 'Out of Africa'. It had alot of low Cs and Ds. I tuned down during the applause before the piece and tuned back up right after. Didn't skip a beat !! The extension would have been nice but I survived without it. The other Basses in the section played 8va as they might get lost and their Basses didn't have the power or equal sound in the lower register as my Martini had.
So Don, what type of 4-string are you looking for.......lol