Queries :: Numeric Comparison And Null Values

I have a form with a textbox, where one inputs a number and then I run a query with the form criteria. The query is

Code:
Select Blah blah from dbo_temp where A>textbox value OR B>textbox value OR C>textbox value OR D>textbox value

The columns which are linked to the textbox some times contain null values. So, A, B, C, D columns do contain null values.

The expression in the "Criteria" column of Columns A, B, C, D (all on different lines - to make sure OR criteria is fullfilled) is as follows

Code:
>IIf(IsNull([Forms]![MainForm]![Criteria]),-100,[Forms]![MainForm]![Criteria])

I've put in -100 as an arbitrary never possible number. Obviously, this does not return Null values.

Question: How can I return both Null & Numbers when the Textbox in the form is left blank? in all the columns. Currently, I am not getting Null values

Code:
>IIf(IsNull([Forms]![MainForm]![Criteria]),SHOW ME EVERYTHING INCLUDING NULL VALUES & NON-NULL NUMBERS,[Forms]![Material Finder]![txtPS])

or in other words

If the textbox is blank, show me all the data available, else if it is not blank then show me only the values that are greater than the number entered in the textbox from within column A, B, C, D ....