Out of memory warnings or errors are caused by the commit charge approaching the commit limit. They have nothing to do with a shortage of available RAM. The commit limit is RAM size + pagefile size - a few MB. With no pagefile it will be close to RAM size. With 4 GB RAM that may not be enough, as appears to be the case.
ReadyBoost acts essentially as a disk cache and is most effective with slow hard drives and limited RAM, generally less than 2 GB. It does not add to RAM, although some articles claim otherwise. It will do nothing for out of memory warnings.
I would question the use of an SD card for the pagefile. It may significantly shorten the life of the drive. SSDs have an efficient form of wear leveling to address this problem but many SD cards have only a simple form. Many do not have it at all.
Typically the pagefile is placed on the system drive. If this causes problems these should be addressed, not by moving the pagefile to some other drive.
Edit: Note that if the pagefile is placed on any removable media you will almost certainly get a system crash if it is removed while Windows is running. There is no safe way to remove such a drive.