Attach a red pigtail to terminals 2 and 3.
Splice source red, pigtail 2, and pigtail 3.
Land load red on terminal 4.
Attach a black pigtail to terminal 1.
Splice source black, load black, and pigtail 1.
Edit:
Note that with this electrical system the hot is red and the neutral is black, not the US color code system.
If you wire this otherwise you can have all kinds of unintended results. If you connect the switch in series with the load in series with the timer, with the timer initially in the "on" interval, it could run until the timer advances to the off position, then both the switch and the timer will turn off. One way to get this odd result would be to wire constant hot / live to 3, NO (4) to load neutral, load hot to terminal 1, and terminal 2 to neutral.
Other hilarity could ensue if you connect the source hot / constant hot to the NO (4) or NC (5).
Since switching neutrals is generally avoided, the neutral should be connected to terminal 2 and no other terminal on the timer switch.
My thoughts would be to very carefully examine the terminal markings and make sure I have identified them correctly, then test the device thoroughly. Also, double check that you have correctly identified the source hot and load hot.
The next test, which the OP has conducted, is to see if the timer runs with source hot to 2 and source neutral to 1. I'd do this again, after double checking the terminal IDs and the source hot.
Next I'd bench test the switch. The instructions don't spell this out explicitly but this is what I'd expect to see assuming it's a typical timer.
With the switch totally disconnected on the workbench, hand turn the timer into the "on" interval you should see
- Continuity - high resistance between 1 & 2
- Continuity - zero resistance between 3 & 4
- No continuity between any other contacts
then hand turn it into the "off" interval and you should see almost the same thing...
- Continuity - high resistance between 1 & 2
- Continuity - zero resistance between 3 & 5
- No continuity between any other contacts
If it passes those tests, and it doesn't operate properly,