I'm an Aquarist so being a Glasscutter has it's advantages

For a tank that size you need to consider the safety coefficient i.e. what thickness of glass you need to hold back the immense pressure from that volume of water.
Lets look at the stats.

1219mm x 762mm x 610mm

That throws up respective approx volumes of; 124.63 UK Gallons (471.72 litres)

Now if you look at the Aquarium Glass Thickness Calculator Chart below, you will see that your tank would need to be constructed from either 9 or 12mm glass, with 12mm giving a higher safety
coefficient of 4.0. (Obviously 12mm plate will be more expensive than 9mm) Balls to juggle!

I would then want to brace this size of tank for added safety, Depending on what you want the tank to be used for (i.e. Freshwater or Marines) would probably determine what way you wished to configure your bracing.
If it were for marines then given the fact that the new Metal Halide pendants for a four footer will have two high powered lamps spaced along its length the a wide central brace bar would be the option to go for as no shadows will be cast from an offending glass spar if it were wrongly positioned.

If it was for Freshwater then a narrower double strut configuration, spaced evenly, may be the way to go. This is of course all dependant on your lighting preferences and the need to negate shadows from spoiling the theme you are trying to create.

For the answer to your main question regarding advantages and disadvantages of embarking on such an adventure then one of the first questions to ask yourself would be "Do I have the confidence in my abilities to execute this project.
The glass will be heavy, even one sheet (the finished article will weigh a great deal) You will need assistance.
If you feel you have what it takes then by all means give it a go.
Be prepared for the odd mishap, sods law with these things, not all run smooth.
But when you've finished and have tested the tank then a great deal of satisfaction can be gleaned from saying "I made that"
I hope this was of some help to you.