Can't speak for a class 4, but at full resolution my class 2 Sandisk started to lose frames after about five or six minutes. No warning was given; I only found out that frames had been dropped upon reviewing the footage when I got home.
Class 4 should be sufficient for video, so I would at least wait and try before shelling out for a class 6 card (or cards). The E-pl1 in general is not a speed demon when it comes to CPU processing, and therefore does not generally take advantage of the speed of a class 6 card in other areas (like writing images to the card) either. Try it out, and then give us a report!
You may be okay with your class 4 cards, but you would have to try it out. I initially bought some cheap class 6 cards for my E-PL1 (Patriot and some other brand), but I was getting corrupt photos. About 1-in-14 photos were corrupted. So I then bought these Panasonic Class 10 SD cards and it solved my problem, and it is fairly fast. It is rated 22mb/s. Plus when I put it in my card reader, it is really fast to download the photos off of it.
This card is made in Japan if that matters to you. Panasonic claims it has some sort of power-failure protection too. If battery goes out while writing a photo, all of your previous photos will still be in-tact.
I've been using a Class 10 Panasonic card for 4 months now and its fantastic! It really comes into its own with continuous shooting JPEG+RAW. If you look around the cost difference with others cards is surprisingly minimal!
I started using Panasonic Class 10 cards too because I like taking multiple exposure shots for HDR processing purposes. My GF1 is usually set up to take 7 exposures at 1/3 increments and the Class 6 cards seem to stall the camera at around shot 5. The performance difference between Class 6 and 10 is really noticeable. The 16 gig cards are often on sale for Can$99.
My sandisk class 6 (4gb) has not failed me and my E-PL1 yet. I have had first hand experience with transcend and patriot cards, and let's just say it was not a good experience (at least I wasn't the one that bought them).
Update for whomever might interest.
I'm getting these dropped frames when, and only when, I do autofocus while recording video on HD.
Luckily for the controlled enviroment where I'm going to use the EPL1, this is not a problem.
I've done some testing with 3 different cards. A transcend class 10 and two transcend class 6, and they all do the same. Sadly I don't have any other brand of cards to do some testing. But it would be great if others could do this test with different types of cards.
I uses Sandisk SDHC Card 16GB class 4 on my E-PL1 and when i recording a HD video format,when it reaches about 600MB or about around 2 minutes,the record suddenly stop,on screen it still had a recording logo on top left corner,but on the time line (right down corner) it shows just "00:00",and hang..i had to push recording button to stop this strange screen..whe i start recording again,it happens again and again.. anyone had a problem like me?is it the speed of the SD Card makes E-PL1 only can write HD Video around 2 minutes length?or may a 16GB capacity on a class 4 makes this look "overloaded"?
i had 8GB Kingston class 4 SDHC and recorded well..
I don't do video, but I do like the fast cards for high speed shooting or to reduce the delay from shooting to preview on my GF1.
I tested my previous card at 13 MB/s write on my PC, and I recently bought a Sandisk Extreme III card that tested at 23 MB/s. I saw a small increase in speed, but not much. So I think that anything faster than 15 MB/s or so is limited by the speed of the GF1.