Single light switch to programmable - 2nd hot line?
The black wire at the bottom of your diagram is the incoming hot feed and goes to all the switches.  If the timer switch has a terminal screw, hook it up to the new switch the same way it was done at the old.  If it has a pigtail lead, you'll have to cut the black wire and wirenut both the cut ends with the pigtail from the timer.  The blue wire or terminal on the timer goes to the other black wire (the non-doubletap one) going to the single switch, and the white wire or terminal on the timer needs to be connected to the bundle of white neutral wires at the back of the box -- you may need to use a length of scrap white wire as a pigtail for this.

Also, the weirdly wired 4-way switch is really a 3 way switch in disguise, and make sure the single black wire's loose end is capped off with a wirenut.