Extending an Airport Express (latest) with a Time Capsule (4th Generation)
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:


No gigabit ethernet connection at the base station (so if a PC is connected to this station and backed up to the Time Capsule, the backup process is capped at ~11 MB/s)
Connecting audio equipment will only be possible at the base station
External HDD can only be attached to the extension, not the base station
802.11n will be provided near the base station and 802.11ac near extension station