My sis's hoosie - around 6-years old - has a retaining 'wall' made of 3" round posts which holds back her neighb's garden which is around 3' higher than hers. Or, it should hold it back - they are all rotten along ground level. You can pull them over with the gentlest pull. He fence is held up with 6x4 posts - even beefier than the 4x4s I normally use. Again, most of them have rotted right through at ground level. Hellish. I'm currently replacing the retaining fence with sleepers as posts - 6 of them with 8x2 planks slipped in behind them. Only 6 holes to make in the ground then... Except that 'Strongvox' - presumably to compensate for using such pitiful round posts - have buried them in a concrete trench, starting around 1 foot under the soil (hence all the rot). What a 'mare. Using a 40J demolition hammer, on my knees, with most of the tool below ground level to cut sleeper-sized holes in 2' of bludy hard concrete. Aaaaaaargh...
Use rsj's as the posts, you can then drop sleepers in, using different width rsj, you can use thinner boards. P:S: Painting rsj's is optional.
I like Committed to using sleepers as the posts now, tho', with thinner 8x2s stacked on edge as the wall behind it (which will also look like sleepers from the front...)