I found I can only add three virtual disks to a guest in a VMware Workstation VM. When I tried to add the fourth disk, the IDE option is disabled automatically.
Does anyone know why?
The limit is actually 4 IDE devices, I am guessing you also have an IDE optical drive on the virtual machine.
As for the reason, this is simply down to the virtual hardware that VMWare Workstation emulates has the standard Primary and Secondary sockets which many motherboards have. Each of sockets supports a Master and Slave device resulting in 4 devices total.
I have tried to do a bit of research, and, I am sure it is technically possible to have more IDE sockets, but, I can't find a single motherboard that shipped with more than 2 - based on this, I am guessing BIOS manufacturers never implemented the ability for more than 2.
IDE also has a lot of restrictions compared to SCSI - so, I am guessing that VMWare added this for backward compatibility reasons only and were never intending people to have many devices.
Here is a picture from the Virtual BIOS (F2 on start).