yeh it is annoying that you cant change weekly, i guess thats part of the challenge.
I dont know what the best rate to set arena prices in div iv because div 3 and 4 can be quite different. my best advice for you would be to have a look at some other div 4 teams and see how much they're setting. It will give you a rough estimate on how much you should set it.
Div 4 prices should be slightly lower than that of div 3. Also it really depends on how your teams going. The better your teams performing the better it is to set prices higher.
If you dont have a winning record i would recommend you set your prices reasonably lower, but if you have a winning record then somewhat higher prices would be ideal.
Hope this helps without actually giving you any hard numbers.
jw