IIRC, the edo ram is texture memory, not video memory. So you won’t find any with 8MB vram.

It wouldn't hurt to store more than textures in the EDO part.
Hypothesis: the VRAM capacity cannot be higher than 4 MB. Hence, why 2 MB cards have 2 MB VRAM expansions, and 4 MB cards have EDO expansions.

It all comes down to what the 2D and 3D cores can do with that memory. I don’t remember off the top of my head what parts can access the edo memory and to what end. The data sheet for the virge vx is available, so shouldn’t be hard to look up.

You should be aware that VRAM is not an entirely different RAM type compared to FPM and EDO, even if it may sound and look like it. Actually, the stuff that makes VRAM special is just a minor add-on to standard DRAM technology. Every VRAM chip made in the 90s is an FPM or EDO memory chip with the VRAM add-on (the serial access memory). The cool thing about VRAM is that it can send data to the RAMDAC at no noticable overhead, while the scan-out from FPM/EDO can use a significant amount of the interface bandwidth of that chip. This is why VRAM chips can provide a huge performance benefit, especially at high resolutions and refresh rates.

Most of the stuff the ViRGE chip does to video memory except for displaying the contents on the screen just uses the FPM or EDO interface. This means you mainly need to have the frame buffer part of the video memory in VRAM, that is the dual-buffered frame buffer in full-screen 3D application, or likely a single-buffered frame buffer in 2D operation. 4MB of framebuffer-capable memory should be more than enough for 3D operation, as the ViRGE/VX 3D performance is barely suitable for 640x480, and makes more sense at resolutions like 512x384. Having just 4MB of VRAM starts hitting 2D limits if you try to exceed 1152x864 at 32bpp or 1600x1200 at 16bpp. If you find software that can program the mode, even 1920x1080 at 16bpp just fits 4MB.

The EDO RAM can be used just as fast as the EDO-based VRAM for memory-to-memory copy operations (DirectX offscreen surfaces) or 3D textures. The drawing engine can perform solid color or pattern fills faster in VRAM than it can in non-VRAM, using a feature called "block writes". As solid color fills and regular dither patterns are more like the 1990s than the 1995 graphics design (which started to be about gradients), the importance of being able to use block writes seems quite low, except for one notorious case: Clearing the Z-Buffer for 3D operation. This means the Z-Buffer can also profit from being located in VRAM, but for all other stuff, I don't expect a practical difference between the VRAM and the non-VRAM EDO parts of the ViRGE/VX memory.