iMac 4,1 does not have Boot Camp update
What you have run into is a fairly common problem with any computer. Once they reach a certain age there stops being updates.

The problem is likely not just Apple. Consider that this Mac was released in 2006, going on thirteen years ago. I do PC/Windows support for a living and we have 5 year old computers with no driver updates available.

It is possible that if you find the make and model of the various components in your iMac (video, sound, network, bluetooth, WiFi, etc.) via Windows Device manager you may find some updates for drivers.

For example if you find that your iMac has a Broadcom L5105 (I made this number up for illustration purposes) chipset, then you would search for Windows 7 drivers for that particular chipset.

But I am betting that you are unlikely to find many (if any) driver updates for 13 year old hardware. Heck Windows 7 itself is end of life in just over a year and Microsoft won't provide any more updates to Windows itself after that.