SQL Query Solution

Question

I have to write a query,in which If I receive a value of

1)length greater than or equal to 4 then I have to add a dot at 4th position and rewrite it.

 2)If it is of length less than four then keep as it is

3)If it already contains dot at 4th position then keep as it is. 

4)but if the dot it is not a 4th position then I have to bring the dot at 4th position and rewrite it.

 

Following is the example of input and output 

 

For example

Value is A456791 then output A45.6791

Value is A456 then  output A45.6

Value is A42 then output A42 

Value is A45.6 then output A45.6 

Value is A.456 then output A45.6

Value is A456.0 then output A45.60

Value is A4567.12 the output A45.6712

 

So far I have written following query

 

declare @str varchar(max)

set @str='A4567891'

if(LEN(@str)>=4)

begin

IF(CHARINDEX('.',@str)>0)

BEGIN

PRINT @STR

END

ELSE

BEGIN

Select SUBSTRING(@str,1,3) + '.' + SUBSTRING(@str,4,LEN(@str))

END

end

else

begin

PRINT 'HI'

end

 

This is working for first 3 points but I am stuck on last point which is  

but if the dot it is not a 4th position then I have to bring the dot at 4th position and rewrite it.

 

That is input is suppose A3119.2 then output should be A31.192

 

How to do this,kindly help me on this