RVsitebuilder version 4 is a very old version that we brought its editor to RVsitebuilder 6 to allow websites continuing. We support in bug-fixes but some are ended of development already, like the problem you're facing now.

The image editor such as resize, still works on Firefox 54 but not be able to in newer versions of browsers.

We have 2 solutions for this case.
1) You can hover/highlight on the image to edit "Source" by defining image's Width and Height as you want.

Or
2) If Firefox on your computer is the compatible one (54), you may stay this old version to edit your website with Resize image function.

But we would like to suggest you to create your website with the new version of RVSitebuilder which will be more functional and most responsive with the latest updated softwares support. 
To create website in RVsitebuilder new version means to start over the whole website as it cannot be transferred.

If you have plan in mind to create website in RVsitebuilder new version, you can start it now in RVsitebuilder 7 by this guide: https://support.rvglobalsoft.com/hc/en-us/articles/360015211433 

==========================================