That's not relevant. Imagine, for example, a car that gets 20mpg costs $10k and one that gets 40mpg costs $20k. It doesn't make them comparable, we need to know how they'll be used. On day 1, the 40mpg car costs $10k more than the 20mpg car. Over time, the difference in the total cost to own for the 40mpg and 20mpg will shrink, and at some point (based on usage) reach a point where the 40mpg car is a cheaper overall investment. Same here.

Gas engine based car: $4,500

Electric engine based car: $20,000

That's not the full price of ownership. At first we need to include fuel, or electricity, so we need estimates of what gas and electricity cost but also how much you plan on driving.

If I drive 2,000 miles/yr, mostly just across town, then I'm going to buy the car because the huge cost for the EV doesn't pay for itself in any time scale I care about.

If I drive 25,000 miles/yr, the huge cost of gas outweighs the cost of the gas-powered car and the EV will pay for itself pretty darn quick.

This is actually even made worse by the fact you could take your $15k in expenditure, if you bought in cash, and invest it to earn ~7%. By buying the $20k EV you give up this opportunity, which costs $1050 and increases each year after that. With loans you have a higher monthly payment for the $20K car and again there's a cost associated with this as long as the difference in car payments would be enough to cover the difference between electricity usage and gasoline usage.

And as a very minor correction, the sales tax on a $20k car is 4x higher than on a $5k car (since it costs 4x as much, I don't mean to imply the sales tax rate is any different, ie, a luxury tax). However, EVs may have offsetting tax credits/deductions that may apply and could potentially make the EV less taxed. In other areas, an annual tax is paid on the value of the car, called an excise tax, in which case the EV would always pay more excise tax - though for a $20k purchase the vehicle is probably valued at $15k by the city/state and one would pay perhaps $300/yr in this tax, decreasing each year.