Hello Karan,
Checking Horizon in terms of ATP checking is completely dependent on your business process as how long in future you would like to see the receipt elements to be considered for the demand to be fulfilled.
Also, your understanding is correct that RLT in ECC is considered as Checking Horizon in APO. Now, for typical RBA scenarios, you might be interested in checking the "Calculation Profile" which is the base for looking for a valid substitute item as per the delay parameter maintained there. It could be blank as well or it could be anywhere in hours or days.
Now, coming to your last question. CH of 4 months. It depends on what type of product is being ATP checked here. Usually CH would be calculated depending on the minimum amount of time it takes to product or procure. That is the reason usually, any demand element beyond CH would not be considered for ATP checking because you have enough time to produce or procure. The problem starts with the demand elements within the CH period and this is the situation where you tend to do RBA or look for any other receipt elements within this 4 months.
Please note that more the CH, system would be generating multiple schedule lines depending on the availability of receipt elements.
Hope this helps
Babu Kilari