Hi
Since it says that the posting period 4 is not open it means July if you are following the Fiscal year as a fiscal year is from Apr - Mar.
Now the from and to work as follows:
If in the from you give a period then that period till whatever period you give in the to field is going to be open for posting. For a better practice it is better to have only one period open.
Hope this clarifies.
In a production system you ought to be very careful while opening periods as it would allow postings in to that period.
Regards