Lets rewrite the sentence so it makes sense:
When I will get home, I will take a long hot bath so that I can relax after a stressful workday.
Although the beginning of the sentence sounds weird, it is technically correct. However, placing a comma after long is attempting to indicate a list of series, but ends after one. In addition, could is for something of the past, whereas can is for the future or present. This means that the 4th sentence is wrong: a long, hot bath so that I could relax
I hope this helps! :)
