The first number is the overall internal size actually. It's the 'Cubic Inches' of space the tank actually holds. With a 50ci tank there are 50 cubic inches.
The 4500 is the pressure rating. So it's rated for 4500 pounds per square inch, psi. Same with a 3000 psi tank, it only is rated to take 3000 psi.
So with the 50/4500 tank you have 50 cubic inches and within 1 cubic inch holds 4500 psi.