Most "common" plecos get large, lazy, and create enough waste that you are basically counteracting any algae-eating abilities they might have...JMO. A 48"x18" footprint is really pushing it for space and unless you really know which species you are getting (darn near impossible with the way most places label plecos) and their potential size beforehand I strongly suggest not buying one.
I suppose you could put them in your pond, however, unless you live in an area where the temp of the pond wouldn't drop below ~70F (they are tropical fish after all) you would need a large enough tank to over-winter them anyways.