When I turn on my Jazz DV 152/ T20 it displays "Read card..." and then nothing happens...I have a 4GB SDHC card by SanDisk...both camera and card are new....is this a faulty camera or am I doing something wrong?

The DV179 uses SD cards, 2GB or smaller. It will not work with higher-capacity SDHC cards. You can use any brand and speed SD card, though personally I'd stick with the major brands.

The problem is that your camera is only compatible with SD cards, not SDHC cards. You bought a larger card for your older camera, not noticing that it was SDHC (or not knowing that it mattered). The new card doesn't work, not because it or the camera is broken, but because it's not compatible.

Cards up to 2Gb are generally SD.
Cards of 4Gb are generally SDHC.
Cards over 8Gb have to be SDHC.

(There are exceptions. Sometimes, manufacturers make 4Gb SDHC cards but some have errors; they disable half of the chip and sell it as 2Gb, but it's still SDHC. 4Gb cards are almost always SDHC because they're cheaper, but 4Gb SD cards do exist. They are so rare that I've even seen a guru on this site claim they don't. I did own one once, but it was very expensive and turned out to be so unreliable that I had to return it. I don't recommend looking for one, because of the problems.)

Your solution is to go find a 2Gb card, and be extra sure that it doesn't say "SDHC" on it.

There are several possible causes for "memory card locked", "memory card error", or "no memory card". Some include trying to use a 4GB SDHC card with an older camera made before SDHC format came out. Others include problems with the tiny slide switch on the side of the card, or the electrical contact switch internal to the camera. The problem may also arise from simple formatting problems if the card has never been formatted in the camera (note that this will also erase the card). Troubleshooting tips for this problem may be found at this link.

if u read your instruction manual it says it only supports up to 4 gb

Your 4GB card is actually an SDHC card. The DX6490 works with SD cards. While SDHC cards will fit into the slot, they are electronically incompatible and will not work. You need to go with SD cards 2GB and smaller.

I read the following spec for your camera: Recording Media Internal Flash Memory 64MB Video/Image
SD/SDHC Up To 32GB Video/Image
So, I think that an 8 GB SD card should work.

Try another capacity and brand to make it work.

Correct!!!! This card reader cannot address cards that are higher then 3gig and cannot read sdhc cards. Most older readers will not be able to read the HC type card, and by older I mean if its 6 months old or more. To read these cards you need to purchase a new card reader. I know, My camera uses 4gig cards and I was unable to access the pics until I bought a new reader..

The ZR200 is able to read a card of maximum of 2 Gbytes !!!

The 4 GB SD card it is a so called SDHC = Secure Digital High Capacity ! Every SD card with more than 2 G Bytes it is a High Capacity and the old hardware will not be able to use these SDHC.

SDHC cards need a special reader. A standard SD reader won't read a SDHC

This can also happen when the lens have an aperture adjustment ring, and it is not set to the "Auto" setting ("A", on my Sigma)/