If diag downloaded correct, so TFTP is fine.
Admin 4500 Stack#$nload address x.x.x.x diag 4000_58001_diag.bin no-reset Downloading Diag Image [\]% Software download not supported on this stack configuration.
For ERS 5.8 software version is only for ERS 4800 as you see in the image file name.
You should verify this information from the release notes:
You may upgrade 4500 devices to 5.7 latest code.
For 5.7.2, download the latest 5.7.2 code from:
Agent code image Secure or normal: Normal is Agent code image 4000_572012.img, 5.7.x and then POE images at the bottom of the download page.
Moreover, diag 4000_57001_diag.bin, 5.7.x Diagnostic image for 4500-PWR+ & 4800 models
4500_53003_diag.bin, 5.7.x Diagnostic image for 4500 models (except 4500-PWR+)
Once done downloading Diag, Image then boot POE and you may also find this in the release notes giving details on upgrade:
If it did not upgrade check, which version is POE on you could go through POE readme 400b15 you need to use 4000_PoEplus_410B4.bin , 5.7.x PoE+ Firmware for 4526T-PWR+, 4550T-PWR+, 4826GTS-PWR+, 4850GTS-PWR+ for 4526-PWR+
For 4548GT-PWR is not the same file.
4500_PoE_400b15.img, 5.7.x PoE Firmware for ERS 4500-PWR (hardware revision 12 or later: 4526T-PWR HW: 12, 4550T-PWR HW: 12, 4524GT-PWR HW: 12, 4526GTX-PWR HW: 12, 4548GT-PWR HW: 12) however this is for Hardware revision 12 that may already be on highest POE.
Once have POE Access points that are working if they are turn on that means it is good to go.