You have use a total of .63Ah from the Battery (3A x 750seconds). This means that you have used approximately 15% of the batteries rated capacity. Unfortunately, the rating of batteries is usually based on a 10 or 20 hour rates, the 4Ah assumes that the draw on the battery is 0.4A (10 hour rate) or 0.2A (20 hour rate) at a current draw rate which would drain the battery in 1 1/3 hours you will not get the full rated energy from the battery.
Still you can the voltage curve of a NiCad to make an estimate. Here is a website that describes the process for a particular battery. Curves may be available for your battery. http://shdesigns.org/batts/battcyc.html