_singleRealTransform4(data, out, outOff, off, step, inv) {
        // radix-4
        // NOTE: Only called for len=8
        const step2 = step * 2;
        const step3 = step * 3;

        // Original values
        const Ar = data[off];
        const Br = data[off + step];
        const Cr = data[off + step2];
        const Dr = data[off + step3];

        // Pre-Final values
        const T0r = Ar + Cr;
        const T1r = Ar - Cr;
        const T2r = Br + Dr;
        const T3r = inv * (Br - Dr);

        // Final values
        out[outOff] = T0r + T2r;
        out[outOff + 1] = 0;
        out[outOff + 2] = T1r;
        out[outOff + 3] = -T3r;
        out[outOff + 4] = T0r - T2r;
        out[outOff + 5] = 0;
        out[outOff + 6] = T1r;
        out[outOff + 7] = T3r;
    }
Computes a single real-valued transform using radix-4 algorithm.
This method is only called for len=8.

@param {Float64Array} data The input data array.
@param {Float64Array} out The output data array.
@param {number} outOff The offset into the output array.
@param {number} off The offset into the input array.
@param {number} step The step size for the input array.
@param {number} inv The value of inverse.