This is an attempt to bring some realism, honesty and a 'can do' attitude to this thread.
Old images won't work with a pi 4 because they won't have the new bootloader firmware or Linux kernel. The Ubuntu packages that deal with these are Linux-firmware-raspi2, flash-kernel and Linux-raspi2. You can check for updates by looking them up on launchpad.net. Wimpy does not/never has maintained these packages. They are spin offs from Ubuntu core. For progress on Ubuntu core see this thread https://forum.snapcraft.io/t/support-for-raspberry-pi-4/11970 . Ubuntu core will also require a new version of u-boot for the pi4 (yet to be written).
Until these packages are in place there is nothing for Wimpy to do. And then it is just the trivial task of re-spinning the images.
For pi4 progress in the generic linux kernel check out this thread https://lists.fedoraproject.org/archives/list/[email protected]/thread/OFIGDPUDFOA3BB33GVCDHTSGZ2Q77APK/
If you want to run Ubuntu on the pi4 now then I suggest you copy the kernel and firmware from Raspbian. This is how ubuntu mate 16.04 used to work. If you don't know how to do this then look up the many threads on getting the 3B+ working.