Are you looking to use this exclusively for Time Machine backups?
With hard drives, there are only three makers left and there is a lot more personal preference than there is verbatim reliability statistics between the different models. If you do not need portability and as you do need capacity, the 3.5-inch form factor makes sense. The G-Technology (Western Digital) drives are solid products, but much like LaCie (Seagate) you do pay a premium predominantly to get a nicer enclosure and sometimes a higher tier hard drive (such as a 7200 RPM drive and/or an enterprise class drive.)
Alternatively, you could purchase a 4 TB 3.5-inch HDD like the WD Blue, and a 3.5-inch aluminum enclosure separately. This is usually a bit cheaper but these enclosures aren't as nice as G-Tech or LaCie enclosures.