this is just sloppy exegesis...
hebrews 4 is talking about resting in christ...and actually the author of hebrews bases his argument on the fact that the seventh day -ended-...that is why it says God 'again fixes a certain day' to enter into God's rest...
There is no mention of the seventh day ending in the Greek...only the cessation of work.
and in genesis 2:4 'in the day' is an idiomatic saying...it is a -completely- different usage from when days are assigned numbers as in 'first day' and 'second day' and so on... you don't number days if they are figurative constructs...
Gen 2.4 shows the flexibility of the term yaum, as it now is used to represent all the previous days as one day.
Assigning an ordinal to the term yaum does not change its meaning according to Hebrew grammar.