What is the limit on selling during your turn?
It is a redundant rule. This printing from Rio Grande Games doesn't even mention the 4 gold maximum per sale. Under Harvesting and selling beans (page 7), it only mentions restrictions on selling from a bean field with fewer than 2 beans only when both fields have exactly 1 bean. The other rule you mention is reiterated though. Therefore, you can earn 8 gold in the example that you gave.


  Players may harvest and sell beans at any time, even when they are not the active player. [...]