The IDE port of the Amiga 4000 is a fully featured buffered IDE port and allows inherently a primary and secondary IDE channel. Two IDE devices can be connected to each of the two channels (master+slave). Please make sure that the devices are configured correctly, as master or slave.
Basically no drivers are needed. However, Amiga OS was primarily designed to manage two IDE devices on the interface. If you run into troubles when connecting up to four devices, please consider to use either IDEfix97 from aminet.net (Amiga OS 1.3 to 3.1) or the new AtapiMagic package (for Amiga OS 3.1.4 to 3.2).