x <- xts(matrix(rnorm(500*8),ncol=8),Sys.Date()-500:1)
x.m <- split(x,'months')

ff <- function(xts.obj){xts.obj[,c(1:4)]}

g <- lapply(x.m, ff)

> args(xts:::`[.xts`)
function (x, i, j, drop = FALSE, which.i = FALSE, ...)
NULL

lapply(x.m, '[', , c(1:4))
lapply(x.m, '[', j=c(1:4))

g <- lapply(x.m, FUN = function(x){x[, c(1:4]}