Will there be a reason not to use an Airport Express (latest model) as the main wifi router and then have the network be extended through an Time Capsule (4th Generation) on bridge mode?
Is the above possible?
The two models differ only in their number of USB and ethernet ports (see https://www.apple.com/compare-wifi-models/ for a comparison of all current models), audio jacks, HDD space and Wifi capabilities. The software features are identical.
Although your proposed setup should be working fine, it has at least the following implications: