For a diamond that size, 4 prong will be just as secure as 6 prong. For a simple solitaire, you can't go wrong with most places. Like DS said, start with where you bought the diamond.
I agree w/ DS & Charmy that the first place to look for settings is with the vendor from whom you purchased the stone.
Since I also have an oval, I wanted to share a setting idea that turned out to work well for me. I prefer 6 prongs (just to give a feeling of additional security). The problem is that the standard 6-prong oval heads place 2 of the 6 prongs at each end of the center stone, thereby making it appear more "pointy" at the ends like a marquise. That bugged me.
So in the case of my new wedding set I worked w/ my vendor (GOG) to configure the six prongs so that they're all located along the sides of the oval. So each side has 3 prongs each & there are no pointy prongs at the ends of the oval.
Charmy's right that you can use 4 prongs if you're comfortable with that. Otherwise I just wanted to share a tip on prong configurations in the event you choose to go w/ 6 instead.
Another vote for getting your setting from your diamond vendor.
I have a six-prong setting with the prongs in the regular configuration. My oval doesn't look like a marquise and the N and S ends of the diamond (which I am likely to knock against things) are protected. Six prongs are more protective and more secure than four.
A setting with customized prong placement probably will not fit the definition of "cheap."