rk4 <- function(f, x0, y0, x1, n) {
    vx <- double(n + 1)
    vy <- double(n + 1)
    vx[1] <- x <- x0
    vy[1] <- y <- y0
    h <- (x1 - x0)/n
    for(i in 1:n) {
        k1 <- h*f(x, y)
        k2 <- h*f(x + 0.5*h, y + 0.5*k1)
        k3 <- h*f(x + 0.5*h, y + 0.5*k2)
        k4 <- h*f(x + h, y + k3)
        vx[i + 1] <- x <- x0 + i*h
        vy[i + 1] <- y <- y + (k1 + k2 + k2 + k3 + k3 + k4)/6
    }
    cbind(vx, vy)
}

sol <- rk4(function(x, y) x*sqrt(y), 0, 1, 10, 100)
cbind(sol, sol[, 2] - (4 + sol[, 1]^2)^2/16)[seq(1, 101, 10), ]

      vx         vy
 [1,]  0   1.000000  0.000000e+00
 [2,]  1   1.562500 -1.457219e-07
 [3,]  2   3.999999 -9.194792e-07
 [4,]  3  10.562497 -2.909562e-06
 [5,]  4  24.999994 -6.234909e-06
 [6,]  5  52.562489 -1.081970e-05
 [7,]  6  99.999983 -1.659460e-05
 [8,]  7 175.562476 -2.351773e-05
 [9,]  8 288.999968 -3.156520e-05
[10,]  9 451.562459 -4.072316e-05
[11,] 10 675.999949 -5.098329e-05