I have an iPhone 4. How can I take manual control over the camera in my phone?
I would love to be able to take manual control over: ISO, Aperture, and Shutter Speed on my iPhone. But does my iPhone 4 even have a variable aperture?
Manual control of the iPhone camera was not added until iOS8, and iOS8 cannot be installed on an iPhone 4. So unfortunately you cannot get the full manual control that you are asking for with your iPhone 4.
iOS8 can be installed on a iPhone 4s but it isn't exactly highly advised as it can reduce performance of the older iPhones(as new iOS releases historically always have) and it also has a reduced feature set.
For those that do have iOS8, quite a few options have appeared since its release that do provide control of ISO and Shutter Speed such as Camera+ and ProCam 2.
It is important to understand that smartphones such as the iPhone have fixed apertures, so you cannot modify them as they are always at their widest setting, regardless of software. If you want to control depth of field, your best bet is to control the focus. See: is it possible to get "bokeh balls" using a cellphone camera?
I would be surprised an iPhone 4 (not a 4s) support advanced camera controls as it does not allow iOS8.
iOS8 offer some additional controls but not in the native camera app; you need to get a 3rd party app like Manual (http://shootmanual.co/).