If the signal is getting through at the wrong frequency, your monitor should flash this up in the screen in its OSD. Going straight into DPMS standby means it's not getting any output at all, not even bad output.
Is it possible it might be the adaptor you use to plug a 9 pin VGA cable into the Amiga's 15 pin video port that is defective? You didn't mention such an adaptor but I guess you use one (I could be wrong, I've never seen an A4000 in my life).
