[R] How to setup the tsp attribute of a dataset

[Samik Raychaudhuri]

Hello,
I am wondering how I should set up the tsp attribute (available through 
attr(x, "tsp")) of a dataset x? Let's assume that x has 100 points, and 
I want to set the frequency to 4.
I tried:
 > attr(x,"tsp")<-c(1,100,4)
Error in attr(x, "tsp") <- c(1, 100, 4) :
  invalid time series parameters specified
Is there any other way to set the frequency of the data?
Thanks.
-Samik