The camera says the sd card is too slow

SOURCE: what is the maximum size sd card for a Pentax

The A10 accepts SD cards, which max out at 2Gb. The 4Gb card is actually an SDHC (Secure Digital High Capacity) card, not an SD card, and thus will not work in devices designed for SD cards. Compatibility goes only one way--SD cards will work in SDHC devices but not the other way around.

SOURCE: When I put in my SD 8 GB memory card, busy signal

An 8GB SD card will actually be a SDHC card, the HC means High Capacity. Your camera is only designed for regular SD cards up to a maximum capacity of 2GB; it's incompatible with SDHC at the basic hardware level and so cannot format the card.

You only have one solution: fit an SD card which does not exceed 2GB. Note that the specification for regular SD cards goes up to 4GB, but very few standard SD devices were ever designed or tested for compatibility with them. The few regular SD cameras which can use them mostly can only do so after a firmware update and even then will operate very slowly compared to when using a 2GB card. There has never been a firmware update to enable your camera to use 4GB SD cards.

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SOURCE: camera works with standard SD memory card, but not SDHC

There are hardware incompatibilities between SD and SDHC cards. They're upward compatible in that SDHC devices will accept SD cards but not the other way around. You will have to hunt down SD cards, 2GB and smaller.

SOURCE: Nikon Coolpix 5900/4 GB Kingston SD Card

The 5900 uses SD cards. The 4GB card you have is actually an SDHC card and will not work in the 5900. You'll have to stay with cards 2GB and smaller.