Question 1
Does a sans-serif typeface that uses an open 4 as a sail number as shown on the picture comply with the rules?

Answer 1
Yes.
The sail numbers shown in this picture satisfy all the requirements of rule G1.2(a):

It should be noted that not all variations of an open 4 will satisfy the last requirement and that legibility shall be assessed case-by-case. For example, numbers or letters created using a typeface resembling a digital 8 display or obtained as a cut-out of a squared number 8 will not comply with it: although they are clearly legible when considered one by one, when they are applied to both sides of a sail they may create confusion, therefore they do not comply with rule G1.2(a).

25 August 2022