Unless you need to veneer 8x4 daily, you have around about 80 square feet of spare workshop space ( remember you need to be able to load it) you have money to burn or just like the general feeling of defeat and failure then go for it!
Making a press of such a size that worked as well as the next best option ( which I will come to in a moment) would basically be financially un viable for a hobby/lite commercial use. Not to mention building one out of wood would be virtually impossible.
Ask yourself how often are you going to be needing to press a whole 8x4. If it's a one off or a small batch run, then it would be far more cost effective to buy you material in already veneered or even arrange for it to be made by a company who specialise in it. I'd go as far as saying even if this cost you £250 a board it would still be the more cost effective solution given the time you would spend making a press then the labour involved in actually pressing.
If you are seriously in need of a press that size then in the grand scheme of things they are relatively cheep to buy (around 30k) considering the amount of work you should have lined up to warrant a press in the first place it would soon re coup its costs.
Now your best alternative is to invest in a vacuum pump and make your bags to suit the size you need. There are plenty of bags out there that you can buy off the shelf but a far superior and cheaper method is to make your own bags up using materials you can buy from any composite supply company. If you really do have a need for a more permanent set up then making yourself a vacuume press table is a very simple and cheep and you are only limited to the size of the spare floor space you have in your workshop.
I would be happy to detail methods and suppliers if you needed me to
Hope this helps.
