If you're like me, initially buying just the bare essentials and building practically everything else around it, you will at various stages along this road want to, if not need to, replace and improve many parts of you printer construction. So it was with my y-carriage, the old one pictured below.
I printed four lm8uu bearing holders (http://www.thingiverse.com/thing:10287). There are many to choose from. I chose these ones because they stand the carriage plate off the rods. That allows the printer platform to compress further onto it's springs, increasing tension and stability, but still overhang either end as it travels.
My older platform had only three bearings. The new one has four. My observation at this point is that while four bearings might seem to be better from a support and stability point of view, it brings alignment challenges you don't have with three. It proved tricky and time consuming to get them all aligned and the carriage moving smoothly before tightening down the bolts. I had to drill slightly oversized bolt holes in the plate to allow fine adjustment.
Conclusion: Moving to a metal base-plate for the y-carriage has increased rigidity and support for the print bed. With this factored in, I'm not sure there was any advantage in going to a four bearing carriage. I think a three bearing carriage is probably as good, is much easier to align, and saves on a bearing!
