I run my HA on my home PC (passive cooling but with an i5 processor, 16GB RAM, an SSD etc.etc.)
It idles (with HA running) at around 16 Watts.
I am guessing a Pi4 (if you include the power pack) will use around 6-7 Watts.
So the difference of 10 Watts will add up to around 87 kWh in one year - which in my case results in an additional cost of around 15.- dollars.
Money well spent in my opinion 
By the way - the TDP of a processor tells you very little about how much your system will use when it is running HA. The only way to find out, is to measure the energy usage of your setup via a multi-meter. a Shelly PM , etc. while executing HA.