Almost every page in a magazine I'm working on fits one of four templates. Those single-page templates are defined in four files. What's the fastest way to create a new page (or replace a blank page) with the contents of one of those templates?
Set up 4 Master Pages.
Then on each master page use
File > Place and place the template file.
Apply the masters where needed.
You won't be able to edit the templates. However, if you go back and change the original template file, the placed versions on the master pages can be updated, keeping them consistent with original template file changes.
If you need to use the templates and edit them, just copy from the template file and
Edit > Paste in Place on the master pages.
I'd recommend managing these four templates out of one InDesign file, moving the content to the master pages, and saving this as an .indt (so it's a true template file). Then, any changes can be made to the master pages of this template and will then be used for all new documents created from this template.
For InDesign documents that have already been created, you can update the master pages.
One way to do this is from the Pages pane to select Master Pages > Load Master Pages.... From there, you can select the document that you want to pull master pages from and whether you want to add these as new masters to your document or have them overwrite the current same-name masters.
Another option, if you want to keep a live link between your template file and the master pages in your documents, is to use the Content Collector tool. With the tool selected, click on the object in your template file that you want as your source. Then go to your other InDesign file and select the Content Place tool. After clicking on this, you'll be given the option to place the object with or without a link back to the original. If you keep the link, then when the original in your template file is modified, the linked object in the other InD file will show a notification that there is an update that's needed to match. You can refresh the link, and it's current with what's in the template.
I'm sure there are several other ways to do this in addition to the ones mentioned here.
If you're new to using master pages in InDesign, I'd recommend digging into documentation on using them. My first go-to is Lynda.com. If you have a subscription, I'd recommend starting with "Creating master pages in InDesign" (http://www.lynda.com/Illustrator-tutorials/Creating-master-pages-InDesign/161817/177374-4.html) and "Creating and applying master pages" (http://www.lynda.com/InDesign-tutorials/Creating-applying-master-pages/368575/412983-4.html).