For cast acrylic use #4, for extruded use #3. Given the thickness I would assume its cast. 40 is almost totally useless IMO, only good for filling gaps which in reality you should never have. I doubt any tank manufacturer uses 40 or something of the like. Also I'd argue using #4 is significantly easier than 40 provided your edges are prepared properly.
You need to use a router or jointer (edge planer) to finish the edges and make them flat prior to gluing/welding and use a syringe with metal tip to apply the solvent. I remember seeing a good post somewhere with someone using a good technique for bubble free seems with thick acrylic. Basically used pins as spacers then flooded the joint with solvent and removed the pins. Personally I've always just balanced the acrylic panel in place and used the syringe with good success but I rarely worked with anything thicker than 3/8".
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