I bought my first home (townhouse) I want to install a smart thermostat I have only one HVAC and the thermostat is located in the main floor. I have noticed that upstairs is hotter than the main floor, ecobee 4 has sensors you can buy for each room. If I only have one furnace is this an overkill since the HVAC will blow air to the whole house?
A true zone system allows different areas of the house to be isolated, by closing off certain areas using dampers, or just having completely isolated air handlers.
This allows fairly accurate control of the temperature each zone, allowing a specific (and if desired, different) temperature to be maintained in each.
The Ecobee is more of a "multiple thermostat" or "zoned sensors" system (not sure if there's a more proper name). The limitation is there is no isolation in air handling, so multiple zones cannot be controlled independently.
However, there are two distinct advantages over the typical "single thermostat" setup:
1) The house can be maintained at a temperature based on averages of all sensors
As an example, let's say right now the upstairs is 26 while the main floor is 21, and your target is 21.
By averaging everything, it'll keep the house slightly cooler and set the upstairs to 23 (a bit above target) while the downstairs is 19 (at bit below target).
2) Which sensors you're using can be scheduled
This means during the day, you can give priority to the main floor living spaces, while at night you give priority to the upstairs.
The downside is during the day the upstairs floor is uncomfortably hot, and during the night the main floor is colder than desired, but the areas where people mainly are stays at a nice temperature.
Aside, I have an Ecobee3 and this works extremely well for me. I also use the "ensure fan runs for at least x min/hour" feature which recirculates air and (I think) keeps everything more even.