The "Mini VGA-to-VGA Adapter" takes on several guises. Be sure
if you happen across one in an online sales page, the seller knows
the differences between the "MiniDisplayPort to VGA" and others.
The part you need would likely be described as: Mini VGA-to-VGA.
While I have an as-new old stock, bought a few years ago from a
thrift-store for $2. that works with my iBook G4 12-inch 1.33GHz
there is no visible part numbers on it that I can tell. At the time, I
also bought three others, same price, that were sealed in bags.
(Those alike, had part numbers; I gave them away, at the time.)
Be also wary of MiniDVI-to-VGA Adapters, as these also may or
may not fit into the similar shaped video-out port; DVI was not a
standard in older PPC Macs, some later PowerBook + G5 iMac
had better later standards than older PPC G4 iMac or G3 iBook.
The same item that fits G4 iBook mid-2005 or before should also
fit the USB2.0 iMac G4 15/17/20-inch. The adapter I have fits my
iMac G4 1.25GHz 17-inch (USB2.0) computer Mini VGA ports.
A look through eBay shows a mix of different parts, and reviews
do show that some sellers have sold more than one type via their
site; some users could not use the one they bought, others could.
The present Apple site does not appear to show any valid older
information regarding these vintage/obsolete adapters any more.
So avoid MiniDisplayPort, MiniDVI, and some versions of Mini VGA
adapters; some versions are of the correct era, but are for PowerBook
G4 models and are a few digits different in part numbers. The plug
may be the correct shape to fit the socket, but lack proper connectors.
Sorry to not be of much help; I did look to see if I could find one of the
old Mini VGA-to-VGA Adapters, here; & only found one w/o part number.
A search brings up almost anything but the correct vintage parts.
The welovemacs.com site had some items but was not clear on what fit
specific models exactly; and their price seemed a bit steep. Other Mac
centric sites may be able to locate vintage adapters. Used ones may be
available, while may also be worn or unreliable.
This page may have some info on various 'new, to old, to obsolete' adapters:
About Apple video adapters and cables - Apple Support
{None of these are the correct ones; so you could use this as an anti-reference}
The one here does not specify iMac G4:
https://www.ifixit.com/Store/Mac/Mini-VGA-to-VGA-Adapter/IF153-030-1
Perhaps if the hour weren't near 2 AM here, I may have better luck searching... 😉