At Six Feet Up, we often get asked the same anxious question: should I upgrade my Plone site now or wait until the next major version comes out (Plone 5 in this case)? Our answer is usually the following: upgrade now to the latest stable version of Plone (emphasis on "stable"). Here's why:
You may be tempted to wait until Plone 5, but anyone on Plone 3 or an earlier dot release of Plone 4 is possibly already at risk (http://plone.org/security/hotfixes). Upgrading to Plone 4.3 today will ensure your mission critical sites stay secure while Plone 5 continues to be developed.
On top of losing Security patches, all versions of Plone before 4.3 are not receiving bug fixes and other development, and will continue to grow farther behind. The sooner your site is on 4.3, the sooner you will be able to take advantage of these features. It will also be a lot easier to extend your site and use new add-ons if needed.
Depending on what add-ons your site needs, Plone 5 may not be compatible with it for some time. It's better to analyze this now and understand what compatibility constraints you will have. Also recent add-ons or add-on upgrades may require tools that are only supported by the latest versions of Plone. For example, the latest version of the FacetedNav product requires jQuery 1.6, which is only supported in Plone 4.2 and above. Plone 4.3 and Plone 5 both support jQuery up to 1.9. Keeping your site on the latest stable version will keep you from running into a lot of these "compatibility lag" issues.
If you are looking for a performance upgrade, Plone 4.3 already offers a nice boost in performance from previous versions (Blog: Why upgrade to Plone 4)
You will not save money by waiting because migrating is a game of stepping stones: you will need to plan on several jumps that take you to the desired version. When you don't have plans to update your site, chances are you will be forced into a last minute emergency migration either due to a security vulnerability, or because a change can't be completed until the site is upgraded.
Our recommendation is to keep your site within that golden window between "too old" and "too new." You don't have to jump on the newest release of any software right away. But don't wait too long that your version is at risk for security vulnerabilities or prevents you from keeping your site fresh. For many reasons, we believe the first step in upgrading to Plone 5 is getting to Plone 4.3.