I have this function:

a <- 1
b <- 2 
get_y <- function (x,a,b) {
   a * b * x
}

And I want to create a function that takes in get_y and returns the x that makes y = 4 for example. How would I do that?

You can solve

get_y(x,a,b) - 4 == 0

with uniroot. You don't have to create a new function, an anonymous one will do it.

a <- 1
b <- 2 
get_y <- function (x,a,b) {
  a * b * x
}

uniroot(\(x) get_y(x,a,b) - 4, c(0, 10))
#> $root
#> [1] 2
#> 
#> $f.root
#> [1] 0
#> 
#> $iter
#> [1] 1
#> 
#> $init.it
#> [1] NA
#> 
#> $estim.prec
#> [1] 8

^{Created on 2023-08-15 with reprex v2.0.2}

Edit

Following ThomasIsCoding's comment, here is a heuristic I many times use to find the search limits.

When the functions are (relatively) simple, use curve to plot the function. If I get it wrong at the first try, expand the limits until I no longer do. In this case, it was at the 2nd try.

# doesn't display the horizontal line,
# the intersection between the function and the line y = 4
# must be outside the plot's y axis range
# curve(get_y(x, a = a, b = b), from = 0, to = 1)

# now yes, there's an intersection, the uniroot 
# search limits can be set to the interval [0, 10]
curve(get_y(x, a = a, b = b), from = 0, to = 10)
abline(h = 4)

^{Created on 2023-08-15 with reprex v2.0.2}