Iberia only operates A340-300 (and -600), but they use the code 342 for their low J configuration of the A340-300. The 342 configuration only has 4 rows of J.

All seats are lie flat with a huge leg space when seated, so in my view it doesn't matter which row you select. Whether you prefer aisle or window is a matter of personal preference. Window seats are more quiet, but you have to climb over someone if you need to get out during the night.