Does 4500x-16s support 10/100 network devices? My vendor says that it is not able to and one of my devices can only support 10/100.
You can use the GLC-T or GLC-TE SFP (1000BASE-T) in your switch which gives you 10/100/1000 copper interface. This SFP supports Category 5, 5e, 6, or 6a UTP/FTP copper cables.
You can see all supported SFP's for the Cisco 4500X here:
Not likely. Cisco list no 100 Mbit modules.
Edit: the 1000BASE-T SFP should be able to link 10/100 Mbit/s.