I don't remember where my copy of "A Clear Track To N Gauge Fun" is which is I think where the N4 plan resides, but...
The typical places to put insulated rail joiners on a double track loop would be in the crossovers. That would allow trains to run independently on each of the loops. Depending on whether the layout is going to be wired for dual cab control (i.e. an Atlas Selector is to be used), you would gap one or both rails. There are differences of opinion on this.
What I wasn't clear on is how many blocks the loops themselves are divided into, and whether the two spurs in the center of the layout were made into another block.