Two is too varied to give a particular answer for.
How many testers do you need? How far from “home” can they be? How much contact do you need with them? Do you need individual reports or just bug submissions? …
Anything from facebook contacts or students at a university to the hundreds of chess organizations around the internet could be the right place to look. It all depends on the specific conditions you apply.
Four is more direct, but it depends on the method you use.
Facebook adds can be relatively cheap, and targeting users whith a “chess”-related interest, but have a somewhat low ‘click’ rate compared to some other options.
Your budget makes the choice for you – since you can make a selection on options based on the budget, but when you have $x to spend you can’t increase it based on the option you like. (or it wouldn’t be a budget)