This is an existing installation that I'm working with.

I want to install a programmable Tstat in place of the existing manual rotary style Tstat.

The baseboard heater is 8 ft long, 240 Volt. I don't know anymore about it.
Coming from the breaker box to the wall junction box there are 4 wires. Grn, Wht, Blk, Red
Then there are 4 more wires continuing on to the heater. The green wires are tied together in the junction box.

It seems to me that I have 2 extra wires in the junction box. (1 to the breaker, 1 to the heater) When I put a meter on the wires coming from the breaker I have black=240V wht=240V and red=110V
I have a 2 pole Tstat (Cadet T4700B) and it seems that I should tie the blk and what 240V lines to it and then put the output to the blk and what lines going to the heater. Is this right?

When I do this the unused wire coming back from the heater to the junction box has 120V on it. Should I terminate this? If so, what's the best way to do this? Or, should it be tied to the 120V line coming from the breaker?

I hope this is clear enough to answer.

Thanks

Where are you measuring the 240 volts across the black and white? If so these should be the 240 volt feed LINE to the stat, and the black and white leaving are the LOAD to the stat. not sure why there is the red, as you say, some sort of spare. You can cap these with wirenuts or electricians tape.