I have standard 2x4 rafters and want to use it just for storage and I am worried they are not strong enough. I was thinking I could buddy ( or sister) up the 2x4's with another 2x4 or 2x6. or stack another 2x4 onto the existing rafter. I am no engineer but logically I am thinking this would work. I am open to any idea that is safe and easy to do.
Depending on the specific structure of your ceiling, this could be a very bad idea.
If done badly, it will simply increase the weight on the structure, not add strength at all.
As a general rule, you should be able to put reasonable storage in the roof space without added effort. (boxes of books ok. crates of lead not ok)
If you have concerns though (and you do, because you're asking), you need to speak to a qualified engineer as each roof is different. (Age, Design, Materials, Climate, Prior Use, Condition, supporting structure are just a few important factors)
Pretty sure you mean "joists," not "rafters." Rafters are the sloped members that run directly under your roof, whereas joists are the horizontal members that sit atop the ceiling (of the floor below the attic)... they create the structural strength of the attic floor.
In regards to your question, however, I'd agree that it depends largely upon what you intend to store. Keep in mind that if you can spread the weight out, a large amount of weight might only mean 1-3 lbs/sqft. I am often putting things into attics that weigh 75-150 lbs, but due to their surface area, they only come out to 2-3 PSF, which is negligible.