Yes, the 4G works well with 3G signals. In addition to an LTE chipset than only work in the US currently, it has a HSPD+ chip that does 3G network.

About the best 3G prvovider, there isn't exactly one 'best' 3G provider. You will have to blance cost , reliability, coverage and availability of Micro-SIMs as you shop for you provider. Safaricom, Orange and Airtel all have 3G in Nairobi and other major cities.

In my experiences, Safaricom 3G is very fast on the new iPad, noticeably faster than iPad 2. but it is also costly. Never tried Orange or Airtel.

Even if we had 4G in Kenya, it wil probably work on the current model iPad because of the difference in frequencies used in USA and what we would use in KE. But future chips may just be dual or quad-band and would work in most countries just like the current generation 3G chips.

You must understand 4G is very new, the chip is large and consumes a lot of power. But by end of this year, this will have reduced in size and power consumption to address multiple frequency bands. But we may still to have commercial 4G then in Kenya.