This equation will always return the week which you are currently in of the current month, in the form of "Week#". Put it in the DefaultSelectedItems field on your combobox.
It takes todays date, finds the difference between itself and the fist day of the month, divides by 7, and rounds up to find the week count. It will also add an additional week if there is the trailing end of a week at the start of the month(i.e if the first day is a thursday, it will account for this shorter week by adding an extra one to the count)
[
"Week" & RoundUp(
DateDiff(
Date(
Year(Today()),
Month(Today()),
1
),
Today()
) / 7,
0
) + If(
Lower(
Text(
Date(
Year(Today()),
Month(Today()),
1
),
"ddd"
)
) = "sun",
0,
1
)
]
However, this equation can produce up to 6 weeks in a month, since it considers leading or trailing part-weeks as weeks, and it looks like you have only 4 weeks as current options?
Something will have to be modified to account for this, either to this equation, or you'll have to make this combobox have more weeks. If necessary, an equation modification should be relatively simple, depending on your specific requirements.
Hope this helps!