Are SD cards created equal?

Pretty much. SD cards have a "nominal" size. 4GB in your case. The actual capacity depends on the implementation of the card and how many bad bits it has (I believe).

Older versions of Win32diskimager will copy from to a smaller card without complaining too much, but that could leave you with a damaged filesystem.

I use a Linux PC to copy my cards by partitioning the new card and copying the contents over with tar, rsync or any other handy tool.

You could use a Linux Live CD to temporarily boot your Windows PC to Linux (it doesn't install any software unless you tell it to), and use the tools on there to copy the contents of your card.

I've been toying with the idea of writing something that can copy cards using just the Pi itself, keyboard, screen, USB card reader, a special script and connecting two pins together on the Pi. It seems promising, but I got distracted by other projects :(

Yes. I use gparted to shrink the partition size before copying from the SD card.

Cheap flash cards can exhibit dubious behaviour from lying about capacity to having uncorrectable bad bits every time you write/read. Try exercising the drives with one of the capacity checking utils like h2testw

Yes.

All 4 GB cards are substantially smaller than 4 GiB (the integral- power-of-2 size). The differences are in the amount set aside for replacing worn out blocks. There is no common specification for how much space is set aside for that purpose. Two cards from different manufacturers are very likely to set aside different amounts.

Dave

USB SDHC adapters exist, so attach one and copy with dd. Don't make things more complicated than they need to be!