Basically it's the FCP version that the FCE version is based on plus all later versions can open the FCE file, kind of like FCP.
FCE1 was based on FCP3
FCE2 was based on FCP4
FCE HD (v3) was based on FCP HD 4.5
FCE3.5 was the universal binary version based on FCP5.1
FCE4 was based on FCP6
There has been no version for FCP7, but has come with speculation that Express has been EOLed.
All the best,
Tom