Partitioning & introducing new boundary points, partitions & filegroup

Hi I am currently studying for my MCTS 70-432.

I am currently working on partitioning data, specifically adding a new boundary point to a new filegroup, I either have not interpreted how a Split Range works, correctly or I am getting the procedure wrong.

I initially have 2 boundary points in my function (Left Range) which is on an int column, & my scheme is obviously on 3 filegroups.

Therefore
Partition1, filegroup 1 = lowerboundary = null & upperboundary = 5

Partition2, filegroup2 = lowerboundary = 5 & upperboundary = 10

Partition3, filegroup 3 = lowerboundary = 10 & upperboundary = null

I first add a new filegroup with file, called Partition4, then add the new filegroup to the partition scheme & finally add the new boundary point 20.

My partitions now return:

Partition1, filegroup 1 = lowerboundary = null & upperboundary = 5

Partition2, filegroup2 = lowerboundary = 5 & upperboundary = 10

Partition3, filegroup 4 = lowerboundary = 10 & upperboundary = 20

Partition4, filegroup 3 = lowerboundary = 20 & upperboundary = null

I don't understand why partition4 is on filegroup 3, I would have expected it to be on filegroup4, I therefore guess I am doing something wrong.

Once I have created the new filegroup and file I run:

ALTER PARTITION SCHEME MCTSPartitionsScheme
NEXT used partition4 ;
go

Then I add the new boundary point:

ALTER PARTITION FUNCTION MCTSPartitionsFunction()
SPLIT RANGE (20);

Any help would be appreciated.
Regards Rob