I was hunting for decent pictures of the E4 and found this site. It has some nice pictures and comments on the E4 and i3 that readers may find interesting.
http://www.pass-thermal.co.uk/flir-thermal-cameras The E4 appears to use a very small lens but it still achieves 0.15 Degrees C sensitivity. Not bad considering. I note that the tiny visible light camera lens is located right above the Germanium lens to reduce the offset error. As Dave states, my external lens would not work on the E4 in its Visible/Thermal combined mode as the visible light camera would be misaligned with the ZnSe lens. You could use it in thermal mode only.
I note that the E4 and i3 are priced the same on the PASS site. A little odd that. I would expect the i3 to be discounted until stocks are exhausted and then discontinued ? The i3 has a fixed focus lens. Not sure about the E4 design but it looks fixed as well. It will be interesting to see what Mike discovers. I am especially interested to see whether some clever interpolation is used to improve the captured image quality. You may think FLIR would include such, but not if it endangers sales of the higher resolution cameras in the range ?
Update: To answer my own questions. The E4 is fixed focus. The E4 example thermal pictures on the FLIR web site show no evidence of interpolation enhancement. They are setting definite image quality boundaries in the range.
http://www.flir.com/thermography/americas/us/view/?id=61194&collectionid=830&col=61195 The price in the USA appears significantly cheaper than in the UK. Not sure whether these units could be purchased when visiting the USA and it is unlikely you could purchase one for export without FLIR preventing such.
Having looked at the specs and prices, I believe the E5 would be my budget minded choice as the E4 falls below what I would consider in terms of resolution.