Read Characters From TextBox

Hi,
I have one text box , textbox limit length is 40 characters.
string one, two,three,four;
i need to read only 10 characters ( 4*10) = 40 characters.
i will get 4 strings i.e 1-10 ( 1st string), (11-20) (2nd-string), (21-30) (3th-string), (31-40) (4th-string);
if textbox contains only 5 characters then it should read 5 characters into string one and remaining strings i need to assing empty.
if textbox contains 15 characters, then it should up to 10 characters to string 1 and remaining 5 characters to string 2 and string 3 and string 4 should be empty.