The iPhone 4 is also not a huge phablet on your bars either. So with the correct case and maybe some mount that puts it out in front of your handlebars or above the stem, it would actually be unobtrusive.

Just a side note, you mention how the iPhone 4 is old tech, bear in mind that the Edge 800 is over four years old and the 810 is over two years old. So my suggestion is use this setup, which by your description is a workable solution, until Garmin announces 'The Next Big Thing.'

@Dave Cutter; your description would be considered normal use for a smartphone, so battery life should never be an issue. Screen time is what sucks battery power and you phone screen is likely off within seconds after placing it in the ziplock and your jersey pocket. That said, Rom5:8 wants to actually use his phone in lieu of a cycling computer, cheap or otherwise.