Not all R4 discs will play on an R2 only machine and vice versa. A lot of R4 packaging however state R2 and R4 (and some don`t, but work) and this is fine (mainly music and some films).
Be careful before buying though, check http://www.michaeldvd.com.au/ for the lowdown on R4 discs as this points out if a disc has dual regional encoding.
Hope this helps,
Anil