C. Guillet, 6 Fantasies à 4 parties (1610) FOR VERSION 2.0

Instruments:

No specific instrument is given; one option could be inferred by the fact the Guillet was an organist and, in the dedication to Carlos de Fonseca, he quotes having played (a first version of?) them at de Fonseca's mansion, presumably on a keyboard instrument.

An element against this is that the work is published in four separate parts: while with a lot of practice (and a wiiiiide music stand!) it is possible to play on a keyboard from 4 separate booklets, it is not the the most reasonable choice if playing on a keyboard was the primary destination.

It should be noted that score layout (as opposed to separate part booklets) for polyphonic pieces was not unheard of; for instance Gardano published in 1577 a score version of Rore's 4-voice madrigals with no text and an explicit destination per sonar d'ogni sorte d'Istrumento perfetto ("to be played with any kind of 'perfect instrument'" that is keyboard). Another example is the Partitura delli sei libri de madrigali a 5 ("Score of the 6 books of 5-voice madrigals") di C. Gesualdo, published by Pavoni (Genova) in 1613. So, if keyboard was the primary intended destination, a score layout would/could have been preferred.

So, another very reasonable option would be any consort of four instruments; we have played a few of them on four viols and they come out nicely enough.
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"Special" time signature:

Have you noted that most pieces have a 'cut C' time signature, but measures contain 4 minims, rather 2? I obtained this by:
1) setting a 4/2 time signature
2) right-clicking on it
3) selecting "Time Signature properties..."
4) checking "Alla breve symbol"

On previous MuseScore version(s), it was possible to reach the same visual appearance, but with more work:
1) setting a 4/2 time signature
2) making it invisible
3) dropping a 'cut C' symbol on top of it from the symbol palette (Z)
4) positioning the symbol at its intended position

the last step was the most cumbersome, as really precise positioning required to open the textual .mscx file, locating the symbol data and editing by hand the X/Y values stored there.

Version 2.0 also helps in this by providing edit boxes where to write precise X/Y offsets for 'nudged' elements: I used this feature to position the small accidentals above the staves.
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Using as a demo:

Thanks for appreciating! As the score is released under the Creative Commons by-sa license, it can be used in almost any way, but I expect more 'demonstrative' demos will appear in the future (or perhaps they exist already).

The specific point in showing this score is that it has been done using ONLY version 2.0, it is not a port from 1.0; and I have to say I suffered remarkably few crashes while creating it.

Thanks to you and to LMRULZ for your words of appreciation!

M.