Bootcamp Failing with Error: "The startup disk cannot be partitioned or restored to a single partition."
Which partition to delete?


Restart your computer and hold down the alt/option key.
Select the recovery partition you wish to keep. For example, with El
Captain OS X 10.11 installed, the recovery partition should be
labeled "Recovery-10.11".
Once booted to this partition, select the Terminal application for
the menu bar.
In the opened Terminal application window, enter the mount
command. The partition you selected will show up in the list. The
other recovery partition will be absent. 


The absent partition is the one to delete.

How to delete it in a (preferably safe) way that would make my hard disk "clean" of everything but my main Macintosh HD partition?

By "clean", I assume you mean you want to remove the extra Recovery HD partition.

If you want to remove disk0s3 then you should first copy the contents of disk0s4 to disk0s3. This can be down using the command given below.

sudo dd if=/dev/disk0s4 of=/dev/disk0s3 bs=16384


You will need to remove the 4th partition. Follow the steps below.


Start your computer from Internet Recovery. Press and hold the
Command-Option+R keys immediately after you turn on your Mac and
hear the startup sound. Your model is capable of using Internet
Recovery, but your firmware may need to be updated. See Computers
that can be upgraded to use OS X Internet Recovery.

Note: An alternative would be to startup using the Snow Leopard (OS
X 10.6) or earlier DVD.
From the menu bar, open the Terminal application.
Enter the following commands in the Terminal application window.

diskutil unmountDisk /dev/disk0
gpt remove -i 4 /dev/disk0