Do you have the Beta version installed?
Android won't let you "downgrade" applications, is just says "Installation failed" or something equally useful.
- You can install the latest beta -- I merge fixes from stable version into betas, you can tell by version name (e.g. beta 1.4.1-47-... has fixes from 1.4.0.47)
- Or join my Google+ community and then leave it, Google Play will downgrade to latest released 1.4.0 (specifically, 1.4.0.30).