How can I import a page from another project into my current project. I inadvertently deleted a page, but have a backup in a backup file.

if you mean to import a page as an sdv4 page, then you can't.

As Wayan says, what was requested cannot be done in SD.

Where the backup file referred to is another project (.rsd) file in the same framework (i.e. Foundation, Materialize etc.), I have done the following to re-create the page fairly quickly.

(a) In the Element Tree of that page of the backup I used the 'Collapse All Nodes' button, added a new container at the top level (I used the class name 'whole-page' for it), and placed all the other items in the Element Tree within that new container.
(b) With that new container selected, in Content > Components I clicked on 'Create Component from Selection', gave that component a suitable name, and clicked on the button to export it to the Library.
(c) In the project file into which I wanted to transfer the page I added a blank page, and in the Content > Components section I used the 'Add Component from Library' button to add the new component to the project file. I then added the component to the blank page.

The above actions take longer to describe than to do: they took just a minute or two. Tidying took somewhat longer as
1. any settings for the Body element had to be be made;
2. the imported elements had many class names and id names changed ('-1' added to the end of them), and I wanted to remove the '-1's.

Item 2 above was made considerable easier than it otherwise would have been by entering (without the inverted commas) '-1' in the search bar at the top of the Element tree. That listed just the elements containing '-1' in the class name or id, and, as each one was corrected, it dropped out of the list automatically.

Frank

PS. I now make a habit of having a 'whole-page' container enveloping everything within the Body of new pages I create. FC

I have done it the same way as Frank described. Works fine!