I have 4-100 watt Renology panels and 2-12 V lithium batteries and a 2000 Watt Renology Inverter.
I am just unsure of what type of controller I should use? Any suggestions?
(I've never done this before but I want to put them on my 6xx10 cargo trailer, but I used to take aircraft engines apart and put them back together so I should be able to do this.)
That will depend on how you intend to wire the array, which in turn may depend on your solar conditions and a number of other factors. Have a look at Victron's MPPT Calculator as well as the free book "Wiring Unlimited" to start getting a good grasp of the factors involved; with extensive mechanical experience as you say, your detail orientation will be an excellent aid to learning about DC electrical systems!
https://www.victronenergy.com/mppt-calculator
https://www.victronenergy.com/upload/documents/The_Wiring_Unlimited_book/43562-Wiring_Unlimited-pdf-en.pdf
