Are these labels self adhering?

Yes, the OL2520 is available on our self adhesive materials which includes the Weatherproof Matte for Inkjet (WJ). You are welcome to request a few free sheets as a sample by visiting the link below. https://www.onlinelabels.com/samples

Will this label fit around a plastic cosmetic jar that is approximately 4 ounces?

Due to the varying sizes and shapes, we'd recommend measuring your jar to confirm that the OL2520 is the right size. You can use our printable ruler or our label size guides to find what will work best for you. https://www.onlinelabels.com/articles/finding-your-label-size-printable-ruler