Can't say why it's running poorly, and it is wise to edit and export a project from the same, or a close version.
But substantial improvements and enhancements have been made since 4.x. Upload a trial version of 8.42 here. Retain your old install file and registration info.
Under OPTIONS | EDITING tab, toggle "use hardware accelerated video effects..." and see if that makes a difference.
Also, before loading a new or long-unsed project, click OPTIONS | DISK tab and Clear Unused Cache Files.
Then load the project and let it fully cache (solid green line above most thumbnails) before beginning to edit.
Also, if you share your sluggish project, someone will be glad to test it on their system. This is easy, quick and can be private. Don't upload the export, just the PORTABLE project folder.
Here's how... http://nch.invisionzone.com/topic/23659-tips-for-getting-help-on-this-forum/
If it contains sensitive material link it in a personal message (PM). Click the envelope above. It won't be passed on or retained. Use FILE|SAVE PORTABLE PROJECT AS, not simply SAVE PROJECT, and don't forget to share it.