Of course it goes without saying that a VPS with 4 CPU's is going to share and run tasks faster than a VPS with 2 CPU's. That is just Computer Science 101. The bigger question is will you notice the increase in speed based on your website.
There are a ton of factors involved with figuring website speed and performance. Here are just some of the top ones to think about ....
1. What type of site are you going to run? Is it a Blog, eCommerce, video hosting, Standard site (if those exist anymore), etc.
2. How big are the pages and what is being processed?
3. What type of content, static or dynamic?
4. How bloated is the front end? Are you writing the front end yourself, or using a out of the box CMS?
5. What type of database if there is one?
6. How many databases if the site is dynamic. (This makes a big difference in performance)
7. Who is managing the databases and will they be normalized. (If you have to ask what that is and this is a big site, with a lot of dynamic data, you need to get a database manager)
8. Is it SSL or standard HTTP. (encryption requires processes and resources)
These are some of the questions that you ask yourself when determining the number of CPU's for a VPS. This is not all inclusive but it covers the high points.
I have seen many customers waste money on multiple CPU's not taking into account what they are doing. Now granted, the additional CPU's are not truly wasted, they do a see a few millisecond increase in performance, but based on the requirements for the site they were designing, that money could have been much more efficiently spent, on a better site design instead of a hardware upgrade.
The would have seen a better (ROI) return on their investment and more bang for their buck with a professional SEO developer or more money on page development or better still database design. For speed on smaller sites (500 pages or less), the database design, or proper database design and normalization will have a better return than most anything else.
Just some thought to think about..... I know it is hard to believe ..... but bigger is not always better.....