sliding scale math formula needed

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mathematics

I have a list of possible values for x. I need a math formula that will spit out certain results depending on the value of x. I could use a list of if/thens, but I was hoping there was a more graceful way of doing it. I need it to weight the values of a search algorithm.

if x = 1 then y = 2
if x = 2 then y = 1
if x = 3 then y = 0.5
if x = 4 then y = 0.25

The closest I've gotten is y = x/(x*2), but that falls apart below 3. I'm gonna keep thinking about it, but if you beat me to it, thanks!