Having Rows With Some Null Values Returned Conditionally

This should be a simple solution, but it has been a long time since I've done any query writing (mostly in Oracle) and I am stumped, so here goes:

We are in the process of converting Access database to MSSQL with web form front ends.

I have a table, all columns are nullable, and want users to be able to query from a form, which has a field for each column and defaults to a % wild card for the entered value.

I want the users to be able to put any string in any field, and have it return each row that matches that, including rows with null values in the other columns, but not the column with the entered criteria.

Here is a sample of the data:

Code:
SQL> select * from test;

COL1 COL2 COL3 COL4
----- ----- ----- -----
this is a test
this is not test
this is not
this is test too
is test too
is too
is too

7 rows selected.

Now, if I have this SQL run, it will return only rows that have no nulls in any columns:

Code:
select
col1,
col2,
col3,
col4from test
where
col1 like'th%'
and col2 like '%'
and col3 like '%'
and col4 like '%';

COL1 COL2 COL3 COL4
----- ----- ----- -----
this is a test
this is not test
this is test too

Now, if I use an OR clause for each column, this mostly works, but the trouble is it will also return rows with null values for the field that has criteria entered in it:

Code:
select
col1,
col2,
col3,
col4from test
where
(col1 like'th%' OR col1 is null)
and (col2 like '%' OR col2 is null)
and (col3 like '%' OR col3 is null)
and (col4 like '%' OR col4 is null);
COL1 COL2 COL3 COL4
----- ----- ----- -----
this is a test
this is not test
this is not
this is test too
is test too
is too
is too

The idea is to only select the first 4 rows in the above example.

I was playing with ISNULL in the select clause, but all it does is substitute a string for a null, and I think CASE will do the same thing.

Is there a way I can write this query so it will return rows with NULL values in any column, except the one(column) that has user entered criteria in it?