Can I use a patented cosmetic ingredient in my new cosmetic line?

without looking at things in detail, the answer is unknown. Go talk with an attorney and go over the details together under attorney-client confidentiality.

The answer to your question depends on whether the patents at issue are valid and infringed. The infringement portion of the analysis requires an detailed analysis of the patent(s) covering the four ingredients "patented by another cosmetic company." More particularly, claims in each of the patents must be compared with your 8 ingredient cosmetic line. If in fact, the other cosmetic company has an open-ended claim on any one of your ingredients, then there may be patent infringement. That you have more unpatented ingredients will not avoid infringement. The validity portion of the analysis may indicate whether the patents in question are enforceable. There are many reasons why a patent may be unenforceable. For example, if the maintenance fees have not been paid, a patent is not enforceable. It may also be the case that the claims allowed were unnecessarily broad and consequently read on the prior art, thereby being invalid. The patent may also be defective for one or more of a number of technical requirements for a valid patent. As you might surmise the analysis of the claims is a critical starting point to making an infringement determination. But, it is also possible that infringed claims are invalid because of prior art or other failed requirements of a patent. Accordingly, you would be well-advised to seek a clearance or noninfringement opinion of counsel before using "a patented cosmetic ingredient" in your cosmetic line, where such an opinion is based on a complete factual scenario and a thorough infringement analysis including prosecution file history where necessary. Such an infringement analysis may also dictate the need for a validity analysis, if there are claims that appear to be infringed.

U.S. Patent Law says that once a patent has been granted, the patent owner has the right to exclude others from making, using, selling, offering for sale, or importing into the U.S. the patented invention during the lifetime of the patent (up to 20 years from filing). However, under the Common Law doctrine of "patent exhaustion" once the patent owner receives compensation for his or her invention through sale of the patented item, the purpose of patent law is fulfilled with respect to that item. In other words, once the patent owner has been paid for the invention, the patent owner's rights to exclude others are "exhausted" and the patent law no longer provides a legal basis for restraining the use and enjoyment of the thing sold. Thus, if you purchase the patented ingredients from their respective patent owners you cannot properly be prevented from using or reselling that patented item in your product.

This is a complex question. From the manner in which you pose it, I can see that have insufficient experience and understand of patents to reach a decision on your own. Here is one of those cases where you should engage an experienced patent attorney (assuming your revenue/potential revenue justifies proceeding with your proposed product line).

Yes, you can use the patented items assuming you can get access to them.