"Did CDROM readers changed file formats?"

The formats haven't changed. The best advice anyone could give you would be to attempt to read the discs in different drives on different computers... you never know, you might get lucky.

This is from the Wikipedia article on CD-Rs. Note the sentence "Unfortunately, some common practices can reduce shelf life to only one or two years."

At present, stated CD-R lifetimes are estimates based on accelerated aging tests as the technology has not been in existence long enough to verify the upper range. With proper care it is thought that CD-Rs should be readable one thousand times or more and have a shelf life of several hundred years. Unfortunately, some common practices can reduce shelf life to only one or two years. Therefore, it is important to handle and store CD-Rs properly if it is necessary to read them more than a year or so later.

Real-life (not accelerated aging) tests have revealed that some CD-Rs degrade quickly even if stored optimally.

Burned CD-Rs suffer from material degradation, just like most writable media. CD-R media have an internal layer of dye used to store data. In a CD-RW disc, the recording layer is made of an alloy of silver and other metals — indium, antimony, and tellurium. In CD-R media, the dye itself can degrade causing data to become unreadable.

As well as degradation of the dye, failure of a CD-R can be due to the reflective surface. While silver is less expensive and more widely used, it is more prone to oxidation resulting in a non-reflecting surface. Gold on the other hand, although more expensive and no longer widely used, is an inactive material and so, gold-based CD-Rs do not suffer from this problem.

Permanent markers are commonly used to mark the label side of CD-Rs and DVDs. This practice has been discouraged because it is believed xylene and toluene, common substances in permanent marker ink, can cause surface deterioration. Additionally, volatile organic compounds may be released which will remain inside the enclosed atmosphere of a CD-R's storage box, causing harm.

One last factor that affects the quality of a CD-R and influences its lifespan is the lacquer that is used to seal the CD-R and protect the dye and the reflective material from the influence of external materials (air, water, alcohol, etc).

I can think of a couple of scenarios why you aren't seeing your files. First, you actually burned them to DVD-r and you're now trying to read them from a CD-ROM drive. Second, you need to refresh Explorer to see the contents of the drive when you've switched discs (just open up your CD drive in Explorer and hit F5 to refresh). Third, the files never copied to the discs at all in the first place. Fourth, those are the wrong discs and you've simply misplaced the correct backups. Fifth, your CD-ROM drive is defective. Sixth, the discs have gone bad or were bad to begin with.

None of the above scenarios sounds all that likely, but they're the only possibilities I can think of. I would suggest checking the discs on a different computer to see if the files are actually on there. But to answer your main question, no there hasn't been a change to CD-r formats in recent years. There are different formats of DVD recordable media, but even they are mostly always compatible on different drives so it's unlikely that would be the cause of your problem.

Good luck!!

There might be two things going on here:
1) Your new CD drive cannot read the type of disk you have burnt to four years ago.
2) Your disc is erased over time

For your sake, I hope it is 1, but written CDs are known to lose their data in about five years, so you are already in the danger-zone.

You can check by trying the CD in another drive, possibly the old one you originally burnt it on. If it work there, you'll know the problem. If it doesn't you still know, but your data will be lost then.