My IPAC hx2790b will not recognise my 4gb sd card

According to the manual, the 610 works with SD cards, which are limited to 2GB in size. It will not work with higher-capacity SDHC cards such as the one you're trying to use.

in the commandprompt:
1.sfc /scannow
2.checkdisk

3.Control Panel - Device Manager - USB Serial Controllers - RIGHT CLICK on EVERYTHING and UNINSTALL
all except the category itself - REBOOT

I dont think the ds is capable of reading sd cards that big. Put it on your computer and reformat your card with FAT32 and then try it. if that doesn't work, try 4gb or 2gb.

format your card and don't rename it will work good luck

Try the 4gb sd card, i have a 730 with iq routes and that done the same

were did you insert it?
try to format it.
or maybe the one that you are using has the defect.

Your slot in the iPAQ is a SD slot, not a SDHC slot - therefore it will not read the card. Your PC card is probably a dual reader. The SD cards look the same but are different. Have a read of this...
http://en.wikipedia.org/wiki/Secure_Digital_card

When You Insert your SD card into the internal card reader, then the SD card is not recognised by the system and the SD card drive doesn't show there. The problem can be because of driver issue, system issue and device issue.
To fix the problem you need to go at device manager if there is a yelllow sign in SD card controller, the problem is most probably caused by a driver issue. Then you need to update the driver. You can also download the driver for your card reader.
Also, check if your SD Card is broken. You can also check it into different devices port.
If it is detected then you can backup of your data.

If the problem not resolved by any means and you require you SD Card data then I will suggest you read - https://www.stellarinfo.co.in/services/removable-media-recovery.php

FYI - Use the SDIO update from the HP site, then do a hard reset and maybe even a couple os soft resets. Interestingly, it takes a while for it to appear (the SD Card). Even if you take it out and put it back, you will need 1 or 2 soft resets to get it to show up.