Transcribing Palestrina

Without knowing the composition you're referring to.
You can go into the staff/part properties of each instrument in the score and uncheck "show time signature". You can use either insert mode or joining/splitting of measure to create them into any custom duration you wish.

If there are no barlines, then how did you assess that there are "like four whole notes to a measure"? But you can in that same staff/part properties dialog also uncheck "show barlines".

It's nothing to do with rubato. Back in those days composers used much longer note values - remember that a group of singers stood round a lectern to read from a single, large choirbook, and so noteheads had to be large. It is quicker to write a large white note than a large black note which needs filling in, particularly with a quill pen. Parts were notated separately, not in score, and barlines weren't used. Instead, mensurations signs indictated how the beat ('tactus') was divided - these evolved into our modern time signatures

Common practice today is to halve or quarter the note values, and write in score with time signatures and barlines. The nineteenth-century editions in Imslp use the original note values, i.e. your whole notes, and barring in 4/2, but modern editions reduce them by half and bar in 2/2. Both retain the original mensuration sign of cut C which is still used today.

The image is of Josquin - I can't find an online source for the original Palestrina but you may have better luck.