Not sure whether this belongs in our HIPs area or our ePO area, but HIPs won out.
Hopefully an expert can help you soon.
Yes, you can check in the HIPS 6.1 package/extension into ePO 4.0, however as you mentioned, the product is EOL and no longer available.
The first thing I would do is try a local uninstall of HIPS 6.1 on the system via Add/Remove Programs (disable Host IPS module first, as usual). If a local uninstall fails to uninstall the software, then a push of HIPS 7.0 install or a HIPS 6.1 uninstall deployment task is not going to work. The software might be in a corrupted state, whereas you will have to manually uninstall HIPS 6.1 off the system, reboot it, and push HIPS 7.0 to it.