The i4 will likely be a sporty, two-door play on the i3 platform, wearing the same carbon fiber and plastic skin as the smaller hatchback. You can expect a wider track for the i4, and likely more power from the electric motors. The optional two-cylinder range-extending engine offered in the i3 will probably make the i4 checklist, too.
The i5 will be a sport sedan akin to the Audi A5 Sportback, and will play off the i8's platform. That means a 1.5-liter turbocharged three-cylinder mill helping electric motors do their thing. Don't expect the i8's 349 horsepower, but the i5 should move along just fine.