Pentax Optio MX4 won't read my new SDHC 4gb

There are several possible causes for "memory card locked", "memory card error", or "no memory card". The most common cause is having the tiny slide switch on the side of the card in the wrong position. The second most common is trying to use a 4GB or higher SDHC card with an older camera made before SDHC format came out. There are also other common causes if the above didn't help, with troubleshooting steps for this problem listed here.

Your memory card is an SD type. These cards have a little switch on the card that can write protect or "lock" it. Take the card out of the camera and check this switch. To unlock it, it should be moved towards the edge of the card which has the electrical contacts.

I have succesfully used 2 GB. I believ that is not the size, but the type of card that may not work. SD will work without problems, but SDHC will not.

You need to format a new card in the camera.

Format the memory on internal and disc.
Vitally

The maximum supported memory for Pentax Optio 430 is 256mb and for Pentax Optio 430 RS is 512mb.

Check this site for more information.

Optio 430
http://www.upgradecomputermemory.com/ram.cfm//memory/P/Pentax/Pentax-Optio-430/

Optio 430 RS
http://www.upgradecomputermemory.com/ram.cfm/memory/P/Pentax/Pentax-Optio-430RS/

It will not work with any "sdhc"

gently tap on the silver ring on lense, while turning on and off the camera, it fixed mine after reading of this fix here on this site

In order to use 4gb memory your camera has to be approve for SDHC use.

If it isn't the most memory you can put into your camera is 2 gb

The maximum supported size is most likely 2GB SD Card. Cards with 4GB and more are following a new (incompatible) standard called SDHC (SD High Capacity), SDHC cards work only in devices specifically supporting SDHC. The Optio S5N uses the old SD standard where 2GB are the maximum size.