Multi-row action for Instead of trigger SQL Server

Scenario/background:

I am trying to create a table of "Tests". For purposes of this question my table will only have 5 columns defined as follows:

 CREATE TABLE TestTable
 (
      Id UNIQUEIDENTIFIER DEFAULT NEWID() NOT NULL,
      Name VARCHAR(75) NOT NULL,
      DateRequested DATETIME NOT NULL DEFAULT GETDATE(),
      TestYear AS YEAR(DateRequested) PERSISTED NOT NULL, -- computed column that shows the year the test was requested. I want to persist this column so I can index on it if need be.
      TestNumber CHAR(4) NOT NULL DEFAULT '0000', -- need this to auto increment but also needs to reset for the first test of the year.
      CONSTRAINT TestTablePK PRIMARY KEY(Id)
 );
 GO

My requirement is that I want the 'TestNumber' to "auto-increment" based on the year. For example:

 GUID, Test 1 in Old Yr, 2013-01-01 05:00:00.000, 2013, 0001
 GUID, Test 2 in Old Yr, 2013-12-25 11:00:00.000, 2013, 0002
 GUID, Test 3 in Old Yr, 2013-12-26 09:00:00.000, 2013, 0003
 ...., ................, ......................., ...., N
 GUID, Test N in Old Yr, 2013-12-31 09:00:00.000, 2013, N+1
 GUID, Test 1 in New Yr, 2014-01-01 11:00:00.000, 2014, 0001   <-- reset to 1

I was thinking that it would be an auto-increment column but how would I reset it based on this being the first test of a new year? So my incorrect solution thus far has been an "instead of insert" trigger defined as follows:

 CREATE TRIGGER InsteadOfInsertTrigger ON dbo.TestTable
 INSTEAD OF INSERT AS
 BEGIN
      -- Get the year of the test request being inserted from the pseudo-insert table.
      DECLARE @TestYear INT;
      SET @TestYear = (SELECT YEAR(DateRequested) FROM inserted);

      -- Grab the maximum TestNumber from TestTable based on the year
      -- that we are inserting a record for.
      DECLARE @MaxTestNumber INT;
      SET @MaxTestNumber = (SELECT MAX(TestNumber) FROM dbo.TestTable WHERE TestYear = @TestYear);

      -- If this is the first test of the year being inserted it is a special case
      IF @MaxTestNumber IS NULL
      BEGIN
           SET @MaxTestNumber = 0;
      END;

      -- Here we take the MaxTestNumber, add 1 to it, and then pad it with 
      -- the appropriate number of zero's in front of it 
      DECLARE @TestNumber VARCHAR(4);
      SET @TestNumber = (SELECT RIGHT('0000' + CAST((@MaxTestNumber + 1) AS VARCHAR(4)), 4));

      INSERT INTO dbo.TestTable(Name, DateRequested, TestNumber)
      SELECT Name, DateRequested, @TestNumber FROM inserted;
 
 END;
 GO

Now here is some DML showing the trigger in action:

 INSERT INTO TestTable(Name, DateRequested)
 VALUES('Some Test', '05-05-2013');
 INSERT INTO TestTable(Name, DateRequested)
 VALUES('Some Other Test', '12-25-2013');
 INSERT INTO TestTable(Name, DateRequested)
 VALUES('Blah Blah', '12-31-2013');
 INSERT INTO TestTable(Name, DateRequested)
 VALUES('Foo', '01-01-2014');
 SELECT * FROM TestTable ORDER BY TestYear ASC, TestNumber ASC;

So as you can see my trigger works for single row inserts but a keen eye will be able to tell it will not work for multi-row inserts.

 CREATE TABLE TempTestTable
 (
      Name VARCHAR(75) NOT NULL,
      DateRequested DATETIME NOT NULL DEFAULT GETDATE()
 );
 GO

 INSERT INTO TempTestTable(Name, DateRequested)
 VALUES('Test1', '01-01-2012');
 INSERT INTO TempTestTable(Name, DateRequested)
 VALUES('Test2', '12-25-2012');
 INSERT INTO TempTestTable(Name, DateRequested)
 VALUES('Test3', '01-01-2013');
 INSERT INTO TempTestTable(Name, DateRequested)
 VALUES('Test4', '01-01-2014');

 -- This doesnt work because it is a multi-row insert.
 INSERT INTO TestTable(Name, DateRequested)
 SELECT Name, DateRequested FROM TempTestTable;

My Question

I realize that I can probably handle this with stored procedures and force users to use stored procedures when updating the tables but I want to be extra careful and prevent sysadmins from being able to do direct inserts to the table with an incorrect 'TestNumber'.

So StackOverflow, my question is how can I achieve this? Would I use a cursor inside of my InsteadOfInsertTrigger? I am looking for alternatives.

Not the neatest thing I'll ever write, but seems to do the job:

CREATE TRIGGER InsteadOfInsertTrigger ON dbo.TestTable
 INSTEAD OF INSERT AS
 BEGIN
    ;With Years as (
        select i.TestYear,
               COALESCE(MAX(tt.TestNumber),0) as YMax
        from inserted i left join TestTable tt
               on i.TestYear = tt.TestYear 
        group by i.TestYear
    ), Numbered as (
        select i.ID,i.Name,i.DateRequested,
               RIGHT('000' + CONVERT(varchar(4),
                  ROW_NUMBER() OVER (PARTITION BY i.TestYear
                                     ORDER BY i.DateRequested,i.Id) + YMax)
               ,4) as TestNumber
        from inserted i
            inner join
            Years y
                on
                    i.TestYear = y.TestYear
    )
    insert into TestTable (Id,Name,DateRequested,TestNumber)
    select Id,Name,DateRequested,TestNumber from Numbered;
 END;

The first CTE (Years) finds the highest used number for each year that's of interest. The second CTE (Numbered) then uses those values to offset a ROW_NUMBER() that's being assessed over all rows in inserted. I picked the ORDER BY columns for the ROW_NUMBER() so that it's as deterministic as possible.

(I was confused about one thing for a while, but it turns out that I can use TestYear from inserted rather than having to repeat the YEAR(DateRequested) formula)