Minimum Memory Requirements: 1GB Windows 10 32-bit, 2GB Windows 10 64-bit
The only way to upgrade to 32-bit Windows 10 is if the previous OS is also 32-bit. You may be able to find a 32-bit version somewhere, but I haven't seen one.
Windows 10 64-bit should run fine with 4GB of memory. One of my laptops has 4GB and it runs without any noticeable performance issues.