possible circuit .....
I have a trial circuit for your light tree ....
The simulation program seems to show that it does what you originally requested:
First step ..... yellow
Second step .... yellow, yellow
Third step .... yellow, yellow, yellow
Forth step ..... green, and all yellows turn off.
It is not perfected yet, one or two bugs... The main problem being that it keeps recycling back to the start after the green light is turned off. I think that problem might be fixed with a transistor ... grounding out the system clock when the green light turns on. That is more of an analog problem, and could not be simulated with the B Spice program.
The only other thing that I'm not certain about is the initialization and reset.
I am guessing that a power off/on cycle will do this job. The first thing to do would be to get a few of the digital logic chips and build a test circuit.
.... Not too many parts are required for this... just some digital ICs, and a few resistors, and capacitors.
Basically, this circuit is just the digital logic that is required for the timing signal.
Once you get the timing to work just right, you can think about the relays and lights. .... Also, you will need to put a few parts together to make a clock generator .... nothing too difficult there.
If you want to take a look, send me a PM ... private message .... with your email. You can download the simulator ..... B-Spice Lite, for free .... and see if you think it will work.
**broken link removed**
I would post the circuit on the forum, but the simulator is not amenable to that. .... So just send a PM.