Hello,
yes, it should be linked against e200z4 libc and librt, primarily due to e200z4 core dual issue hardware.
e200z650 is single issue. If an executable is linked with e200z6 libraries, the code will work on PXS20 because the e200z4 instruction set is compatible with e200z6. However, e200z4 libraries are built with compiler support for dual issue - means the compiler should generate more optimized code for dual issue hardware. We will fix MQX PXS20 projects for this.