I'm kind of on vacation from the phono-hobby right now, but thought I'd give a quick response here, since this is something I have some personal experience with.
The 4-40 is a very nice performer. Though it does not have quite the bass of a Credenza, it pumps out the same, full, mellow tone, and with a properly restored Orthophonic sound box, provides an amazing, room-filling sound. Like the Credenza, the 4-40 is fitted with a four-chamber Re-Entrant horn, illustrated in the last post in this string --
viewtopic.php?f=7&t=258 It's really quite an improvement, in terms of range, over the Granada model, which had a much smaller horn.
I've played mine with both a carefully restored HMV 5a sound box (virtually the same as the 5b, in terms of performance), and a good condition Orthophonic sound box. Hands down, the Orthophonic sound box is the winner. While the HMV 5a will provide the same sonic range, it does not project the sound into the room the way the Orthophonic sound box does. The volume is also slightly reduced with the 5a.
Since any Orthophonic sound box will fit your tonearm, I'd try to hunt down a good condition example--even the type used on the portable models such as the VV-2-55 or 2-60, though avoid the one used on the VV-2-35, which was a different design. (They normally cost about the same as a good condition 5a/b version, based on what I've seen on eBay, etc.)
As a stop-gap measure, you might want to try a quick overhaul of the Orthophonic sound box you have, unless, of course, there are major cracks in the front or back plate. The major cause of rattling is a poorly seated needle bar bearing. You can fix this by following the instruction posted on this page --
http://www.gracyk.com/orthosound.shtmlThis, of course, assumes that the diaphragm is not damaged and that the needle bar is properly soldered in place.
I'd also recommend putting a few drops of something called "Goo Gone," which is used to remove sticky gunk left over from price tags, etc, around the edge of the diaphragm, and swish it around the edge. This will cause the paper-backed rubber gasket to swell somewhat and seal any gaps.
As for the 4-40, itself, when you have a chance, go over the entire tone chamber and make sure there are no leaks or seam gaps, which can lead to a loss of performance, especially bass.
HTH,
Joe_DS