I literally wrote this passing out. Seems to work. I've got a bunch of files in the format of something/yyyy-mm-dd-... and I want to make somethingelse/{yyyy}.tar files out of them. This does it.

ptar.go

package main

import (
	"archive/tar"
	"flag"
	"io"
	"log"
	"os"
	"path/filepath"
)

func maybefatal(err error) {
	if err != nil {
		log.Fatalf("%v", err)
	}
}

func addFile(w *tar.Writer, filename string) {
	f, err := os.Open(filename)
	maybefatal(err)
	defer f.Close()

	fileSize, err := f.Seek(0, 2)
	maybefatal(err)
	_, err = f.Seek(0, 0)
	maybefatal(err)

	w.WriteHeader(&tar.Header{
		Name: filename,
		Mode: 0666,
		Size: fileSize,
	})

	_, err = io.Copy(w, f)
	maybefatal(err)
}

func process(base string, tarfiles, fns []string) error {
	log.Printf("Processing %v files", len(fns))

	prefixes := map[string]*tar.Writer{}
	for _, n := range tarfiles {
		b := filepath.Base(n)[:4]

		f, err := os.Create(n)
		maybefatal(err)
		defer f.Close()
		w := tar.NewWriter(f)
		defer w.Close()

		prefixes[b] = w
	}
	log.Printf("Tarfiles: %v", prefixes)

	for _, n := range fns {
		w := prefixes[n[:4]]
		if w == nil {
			log.Printf("Skipping %v", n)
			continue
		}
		log.Printf("Doing %v", n)

		addFile(w, filepath.Join(base, n))
	}

	return nil
}

func main() {
	flag.Parse()

	d, err := os.Open(flag.Arg(0))
	maybefatal(err)

	names, err := d.Readdirnames(0)
	maybefatal(err)

	process(flag.Arg(0), flag.Args()[1:], names)
}