A P4 motherboard won't be AT. AT mobos went out around the PentiumII era. So long as it looks like the dimensions fit so the motherboard can physically fit in the case, you should be fine.
The most difficult thing for you will probably be connecting the front panel switches to the proper pins on the front panel header block on the motherboard. There are pairs of wires for each of these front panel devices: Power switch, reset switch, power LED, hard drive LED, system speaker (makes beeps). If the motherboard doesn't have printed text next to this block of pins identifying which pair of pins is which, it's going to be an adventure of discovery for you to try and decipher which wires go where.