You can also just use electrical tape to bundle (3) AA batteries together, and then solder a wire across the bottoms for connection to the black battery lead and do the same across the tops of the batteries, for connection to the red battery lead. Obviously, a plastic battery holder into which you can insert the batteries would be easier. Have you removed the VRAM card and cleaned its gold-plated or tinned contacts, front & back? Oxidization can occur on them, causing problems such as you're experiencing. Do you know which type of VRAM card is installed in the slot (next to the left side of the computer)? It could be a 1 or 2 MB EDO DIMM or a 1, 2, or 4 MB SGRAM DIMM.
I suppose that you have no idea if this 4400 was functioning correctly, the last time the owner used it? For better graphics, I'd suggest installing a Mac-compatible PCI graphics card The Apple/ATI Rage 128 found in the Blue & White G3s is a good choice, and it has 16 MBs of VRAM. The PC versions of any of these older graphics cards won't function in your 4400, because they must be programmed with a Mac ROM. More than 10 years ago, I bought several ATI Rage Pro (Mac) PCI graphics cards for use with my older Power Macs. The cards had 4 MBs of onboard SGRAM and a slot to accommodate ATI's proprietary 2 or 4 MB SGRAM card. I typically installed a 4 MB card, to provide 8 MBs of SGRAM. It was a nice upgrade, over the onboard GPUs found in those models. Check all of the memory DIMMs, to be sure that they're compatible: 64-bit wide, 168-pin unbuffered 3.3-v EDO (60ns), with a 1K or 2K refresh rate (4K isn't supported). Is an L2 Cache card installed in the slot? If so, make sure that it and the memory DIMMs aren't oxidized on the pins. If so, gently clean them with a soft (white) eraser and blow off any rubber shavings, prior to re-installation.