add tunings for mu=4

Adds [600,650,700,1000]KHz threshold tunings for mu=4, accounting for changes in the seeding, standard efficiency plots follow:

600KHz

650KHz

700KHz

1MHz

I haven't added sequences for these but can; do we want the mu tuning in the sequence name (I would not, from the pov of already quite some proliferation of flags and settings / here, and instead just update the existing sequences to use these tunings)

@decianm @cmarinbe