It's possible to insert custom segments into the time headers cells using time-based active areas.
This technique is used in the following demo to split a month into thirds:
http://javascript.daypilot.org/demo/scheduler/timeheaderactiveareas.html
dp.onBeforeTimeHeaderRender = function(args) {
if (args.header.level === 1) {
var point1 = args.header.start.addDays(10);
var point2 = args.header.start.addDays(20);
args.header.areas = [
{start: args.header.start, end: point1, top: 0, bottom: 0, backColor: "rgba(0, 255, 0, .4)", html: "1st third"},
{start: point1, end: point2, top: 0, bottom: 0, backColor: "rgba(255, 0, 0, .4)", html: "2nd third"},
{start: point2, end: args.header.end, top: 0, bottom: 0, backColor: "rgba(255, 255, 255, .4)", html: "3rd third"},
];
args.header.html = "";
}
};
You can use the same technique to split a day into 4-hour blocks.