Of course, you won't simply say, "I want to create four subdomains." Deciding how many child domains to implement is really choosing the organizational affiliation of your subdomains. For example, if your company has four branch offices, you might decide to create four subdomains, each of which corresponds to a branch office.
Should you create subdomains for each site, for each division, or even for each department? You have a lot of latitude in your choice because of DNS's scalability. You can create a few large subdomains or many small subdomains. There are trade-offs whichever you choose, though.
Delegating to a few large subdomains isn't much work for the parent domain, because there's not much delegation to keep track of. However, you wind up with larger subdomains, which require more memory and faster name servers, and administration isn't as distributed. If you implement site-level subdomains, for example, you may force autonomous or unrelated groups at a site to share a single name space and a single point of administration.
Delegating to many smaller subdomains can be a headache for the administrator of the parent. Keeping delegation data current involves keeping track of which hosts run name servers and which zones they're authoritative for. The data change each time a subdomain adds a new name server, or when the address of a name server for the subdomain changes. If the subdomains are all administered by different people, that means more administrators to train, more relationships for the parent administrator to maintain, and more overhead for the organization overall. On the other hand, the subdomains are smaller and easier to manage, and the administration is more widely distributed, allowing closer management of subdomain data.
Given the advantages and disadvantages of either alternative, it may seem difficult to make a choice. Actually, though, there's probably a natural division in your organization. Some companies manage computers and networks at the site level; others have decentralized, relatively autonomous workgroups that manage everything themselves. Here are a few basic rules to help you find the right way to carve up your name space:
Don't shoehorn your organization into a weird or uncomfortable domain structure. Trying to fit 50 independent, unrelated U.S. divisions into 4 regional subdomains may save you work (as the administrator of the parent zone), but it won't help your reputation. Decentralized, autonomous operations demand different domains - that's the raison d'Ítre of the Domain Name System.
The structure of your domain should mirror the structure of your organization, especially your organization's support structure. If departments run networks, assign IP addresses, and manage hosts, then departments should manage the subdomains.
If you're not sure or can't agree about how the namespace should be organized, try to come up with guidelines for when a group within your organization can carve off their own subdomain (e.g., how many hosts do you need to create a new subdomain, what level of support must the group provide) and grow the namespace organically, only as needed.