An ISO file is a digital archive of a UMD. Since Sims 4 was never pressed onto a UMD for the PSP, no legitimate ISO exists anywhere on the internet. If you find a website offering a “Sims 4 PSP ISO” download, you are almost certainly looking at one of three things:
Downloading ISOs of games you do not own is considered piracy. However, if you physically own the UMD, creating a personal backup ISO (via custom firmware and a UMD ripper) is generally accepted in many jurisdictions as fair use for archival purposes.