How to split a string variable in variables in R (non-rectangular)

I have the following variable in a dataframe

test<-data.frame(x=c("", "1-7-9", "3", "2-4-6-8"))

I want to splits that in variables like:

Var1 Var2 Var3 Var4
NA   NA   NA   NA
1    7    9
3    NA   NA   NA
2    4    6   8

I have tried

 test2<-strsplit(as.character(vartest$x), "\\-")  

but I get a list rather a dataframe

Please help me