I have an iMac 4,1, 2Gb RAM and installed Bootcamp. Is Windows 10 Home Premium 32 bit good for that machine? Or is it too heavy, better install Windows 7 Home Basic 32 bit?
The latest supported Windows version for that machine is Windows 7, 32-bit.
There will be no drivers available for Windows 10.
Your Mac meets the minimum requirements to run Windows 7 and 10. Generally, Windows 7 (SP1) requires a product key to download and install. Windows 10 does not. In either case, if you wish to keep Windows after a trial period, you will need to purchase a license. OS X does not have to be installed on the Mac before installing or using Windows, provided you have the Snow Leopard DVD. You could also install both versions of Windows as a dual boot. This way you could boot each and compare speed and features. With dual boot, you first install Windows 7 in one partition, then Windows 10 in a different partition. The Windows 10 installer will automatically setup the dual boot software.
Since Windows 10 did not exist when Snow Leopard was released, you will probably need to run the Boot Camp Support Software installer in Windows 7 compatibility mode. If this still does not work, then consider first installing Windows 7, then the Boot Camp Support Software, and finally upgrading to Windows 10.
When running a newer version of Windows that is not officially supported by Apple, you will often find the Device Manager will show errors. Usually, these errors are generated by redundant hardware that is not used anyway. One solution is to provide a null driver which effectively just removes the errors from being displayed in the Device Manager. If your Mac appears to work properly, then the simplest solution is just to ignore the displayed errors. This problem is not unique to Macs, the same is true for all other of machines capable of running Windows.