diff --git a/lz4.go b/lz4.go
index 68fecaa7..3d6b0a21 100644
--- a/lz4.go
+++ b/lz4.go
@@ -16,6 +16,8 @@ type Lz4 struct {
 // Compress reads in, compresses it, and writes it to out.
 func (lz *Lz4) Compress(in io.Reader, out io.Writer) error {
 	w := lz4.NewWriter(out)
+	// TODO archiver v4: use proper lz4.Fast
+	// bitshifting for backwards compatibility with lz4/v3
 	options := []lz4.Option{
 		lz4.CompressionLevelOption(lz4.CompressionLevel(1 << (8 + lz.CompressionLevel))),
 	}
diff --git a/tarlz4.go b/tarlz4.go
index 1bad3e7a..42cbc90b 100644
--- a/tarlz4.go
+++ b/tarlz4.go
@@ -84,6 +84,8 @@ func (tlz4 *TarLz4) wrapWriter() {
 	var lz4w *lz4.Writer
 	tlz4.Tar.writerWrapFn = func(w io.Writer) (io.Writer, error) {
 		lz4w = lz4.NewWriter(w)
+		// TODO archiver v4: use proper lz4.Fast
+		// bitshifting for backwards compatibility with lz4/v3
 		options := []lz4.Option{
 			lz4.CompressionLevelOption(lz4.CompressionLevel(1 << (8 + tlz4.CompressionLevel))),
 		}