what battery to use.

[BrianG]

4Ah @ 15C is only 60A. Most people shoot for at least 100A for 8th scale stuff. A 60A current rating would be ok if using 5s or better yet, 6s on a ~1700kv motor geared ~35mph or less. Using a 60A rated battery on 4s may cause the voltage to dip tripping the LVC, and/or cause the battery to work too hard.

Now, if you ran two of those in parallel, you'd be much better off IMO. 8Ah @ 15C is 120A.