Dear R users,
i have the data in the following format and i want to convert it in the format given at the end.
Ta ans Sa are the names of certain cities. there are 69 cities in my data.
column 1 is representing station name (i am writing the data of only two cities for simplicity but actually, as i wrote, i have 69 cities and the actuall table runs down very deep.) 

Column 2 represnts the year for which the data is given (Actuall data for each station is of different length but atleast of 24 years).

Column 3 and 4 reprent the month and the day of the data. obviously each year has 12 months and each month as different number of days, but to make table easily understable only 3 months and 3 days of each month are considered. febrary for leap years should also be considered.

col5	represents population of that city

st.	year	month	day	population in million
Ta	1966	1	1		2.4
Ta	1966	1	2		2.4
Ta	1966	1	3		2.4
Ta	1966	2	1		2.4
Ta	1966	2	2		2.4
Ta	1966	2	3		2.4
Ta	1966	3	1		2.3
Ta	1966	3	2		2.2
Ta	1966	3	3		2.3	
Ta	1967	1	1		2.4
Ta	1967	1	2		2.4
Ta	1967	1	3		2.4
Ta	1967	2	1		2.4
Ta	1967	2	2		2.4
Ta	1967	2	3		2.4
Ta	1967	3	1		2.4
Ta	1967	3	2		2.4
Ta	1967	3	3		2.4
Ta	1968	1	1		2.4
Ta	1968	1	2		2.4
Ta	1968	1	3		2.4
Ta	1968	2	1		2.4
Ta	1968	2	2		2.4
Ta	1968	2	3		2.4
Ta	1968	3	1		2.3
Ta	1968	3	2		2.2
Ta	1968	3	3		2.3	
Ta	1969	1	1		2.4
Ta	1969	1	2		2.4
Ta	1969	1	3		2.4
Ta	1969	2	1		2.4
Ta	1969	2	2		2.4
Ta	1969	2	3		2.4
Ta	1969	3	1		2.4
Ta	1969	3	2		2.4
Ta	1969	3	3		2.4
Sa	1955	1	1		2.4
Sa	1955	1	2		2.4
Sa	1955	1	3		2.4
Sa	1955	2	1		2.4
Sa	1955	2	2		2.4
Sa	1955	2	3		2.4
Sa	1955	3	1		2.3
Sa	1955	3	2		2.2
Sa	1955	3	3		2.3	
Sa	1956	4	1		2.4
Sa	1956	4	2		2.4
Sa	1956	4	3		2.4
Sa	1956	5	1		2.4
Sa	1956	5	2		2.4
Sa	1956	5	3		2.4
Sa	1956	6	1		2.4
Sa	1956	6	2		2.4
Sa	1956	6	3		2.4
Sa	1957	7	1		2.4
Sa	1957	7	2		2.4
Sa	1957	7	3		2.4
Sa	1957	8	1		2.4
Sa	1957	8	2		2.4
Sa	1957	8	3		2.4
Sa	1957	9	1		2.3
Sa	1957	9	2		2.2
Sa	1957	9	3		2.3	
Sa	1957	10	1		2.4
Sa	1958	10	2		2.4
Sa	1958	10	3		2.4
Sa	1958	11	1		2.4
Sa	1958	11	2		2.4
Sa	1958	11	3		2.4
Sa	1958	12	1		2.4
Sa	1958	12	2		2.4
Sa	1958	12	3		2.4
...
...
uptill 69th station



i want to convert the data in following format

>Ta ## matrix for station Ta
1966			1967			1968			1969
AVERAGE	OF MONTH 1	AVERAGE	OF MONTH 1	AVERAGE	OF MONTH 1	AVERAGE	OF MONTH 1
AVERAGE	OF MONTH 2	AVERAGE	OF MONTH 2	AVERAGE	OF MONTH 2	AVERAGE	OF MONTH 2
AVERAGE	OF MONTH 3	AVERAGE	OF MONTH 3	AVERAGE	OF MONTH 3	AVERAGE	OF MONTH 3
........
........
AVERAGE	OF MONTH 12	AVERAGE	OF MONTH 12	AVERAGE	OF MONTH 12	AVERAGE	OF MONTH 12

similar operation are to be done for "Sa" and the remaining 67 stations...
which means i want to have 69 matrices, in which each column (number of columns should be equal to number of years of data)  should contain 12 mean monthly values of each year.