_realTransform4(out, data, inv) {
        // Real input radix-4 implementation
        const size = this._csize;

        // Initial step (permute and transform)
        const width = this._width;
        let step = 1 << width;
        let len = (size / step) << 1;

        let outOff;
        let t;
        const bitrev = this._bitrev;
        if (len === 4) {
            for (outOff = 0, t = 0; outOff < size; outOff += len, ++t) {
                const off = bitrev[t];
                this._singleRealTransform2(data, out, outOff, off >>> 1, step >>> 1);
            }
        } else {
            // len === 8
            for (outOff = 0, t = 0; outOff < size; outOff += len, ++t) {
                const off = bitrev[t];
                this._singleRealTransform4(data, out, outOff, off >>> 1, step >>> 1, inv);
            }
        }

        // Loop through steps in decreasing order
        const table = this.table;
        for (step >>= 2; step >= 2; step >>= 2) {
            len = (size / step) << 1;
            const halfLen = len >>> 1;
            const quarterLen = halfLen >>> 1;
            const hquarterLen = quarterLen >>> 1;

            // Loop through offsets in the data
            for (outOff = 0; outOff < size; outOff += len) {
                for (let i = 0, k = 0; i <= hquarterLen; i += 2, k += step) {
                    const A = outOff + i;
                    const B = A + quarterLen;
                    const C = B + quarterLen;
                    const D = C + quarterLen;

                    // Original values
                    const Ar = out[A];
                    const Ai = out[A + 1];
                    const Br = out[B];
                    const Bi = out[B + 1];
                    const Cr = out[C];
                    const Ci = out[C + 1];
                    const Dr = out[D];
                    const Di = out[D + 1];

                    // Middle values
                    const MAr = Ar;
                    const MAi = Ai;

                    const tableBr = table[k];
                    const tableBi = inv * table[k + 1];
                    const MBr = Br * tableBr - Bi * tableBi;
                    const MBi = Br * tableBi + Bi * tableBr;

                    const tableCr = table[2 * k];
                    const tableCi = inv * table[2 * k + 1];
                    const MCr = Cr * tableCr - Ci * tableCi;
                    const MCi = Cr * tableCi + Ci * tableCr;

                    const tableDr = table[3 * k];
                    const tableDi = inv * table[3 * k + 1];
                    const MDr = Dr * tableDr - Di * tableDi;
                    const MDi = Dr * tableDi + Di * tableDr;

                    // Pre-Final values
                    const T0r = MAr + MCr;
                    const T0i = MAi + MCi;
                    const T1r = MAr - MCr;
                    const T1i = MAi - MCi;
                    const T2r = MBr + MDr;
                    const T2i = MBi + MDi;
                    const T3r = inv * (MBr - MDr);
                    const T3i = inv * (MBi - MDi);

                    // Final values
                    out[A] = T0r + T2r;
                    out[A + 1] = T0i + T2i;
                    out[B] = T1r + T3i;
                    out[B + 1] = T1i - T3r;

                    // Output final middle point
                    if (i === 0) {
                        out[C] = T0r - T2r;
                        out[C + 1] = T0i - T2i;
                        continue;
                    }

                    // Do not overwrite ourselves
                    if (i === hquarterLen)
                        continue;

                    const SA = outOff + quarterLen - i;
                    const SB = outOff + halfLen - i;

                    out[SA] = T1r - inv * T3i;
                    out[SA + 1] = -T1i - inv * T3r;
                    out[SB] = T0r - inv * T2r;
                    out[SB + 1] = -T0i + inv * T2i;
                }
            }
        }

        // Complete the spectrum by adding its mirrored negative frequency components.
        const half = size >>> 1;
        for (let i = 2; i < half; i += 2) {
            out[size - i] = out[i];
            out[size - i + 1] = -out[i + 1];
        }
    }
Real input radix-4 implementation
@param {Float64Array} out Output array for the transformed data
@param {Float64Array} data Input array of real data to be transformed
@param {number} inv The scale factor used to normalize the inverse transform