How to remove disk0s4 partition?

Hello,

I recently split my internal harddrive into two partitions in order to install Windows 8.1, as bootcamp was not working for me.

I had Windows up and running but had to delete it temporarily - I did this by deleting the bootcamp partition in disk utility (I'm new to this so apologies if this was a stupid thing to do)

Now the bootcamp partition is free space, but this disk0s4 partition is present and I just can't remove it - I need to as I want to install Windows 8.1 again.

- I have tried recovery and have had no luck.

- I tried reinstalling Windows, but I get a message saying Windows doesn't have the necessary boot drivers.

These are my specs:

Mac (21.5-inch, Mid 2011)

2.5 GHz Intel Core i5

4 GB 1333 MHz DDR3

Macintosh HD

AMD Radeon HD 6750M 512 MB

Yosemite 10.10.1

Please, I have been trying for days on end and am desperate for a solution.

Thank you,

Any help greatly appreciated.