The 300g archerfish paludarium that oneshot linked to earlier was mine, so hopefully I can offer some relevant insights.
First off, archerfish are awesome! I've kept various kinds of fish for a long time, and that paludarium was definitely the most fun setup I've ever had. I was going to point out that you make sure to get T. microlepis for a freshwater setup (or T. blythii if you have very deep pockets) but it sounds like you know that already.
In my opinion, you should try to get at least 5 or 6 because their dynamics are much more interesting in a school - the more the better. That being said, it sounds like you're only going to have 4'X1'X1' body of water, which isn't very much. When you design your tank, try to maximize the water area - e.g., consider making the land part an overhang, so that there's additional water underneath it.
Arhcers (even microlepis) do get quite large and are powerful swimmers and jumpers. They also need VERY clean water - I found that slight drops in water quality would cause them to lose their appetites or develop cloudy eyes. They also seemed to be very sensitive to chlorine/chloramines in tap water, so be sure to use a good dechlorinator during water changes.
As you know, I decided not to have any animals in the terrestrial portion of my tank. Once the archers settled in, they were very enthusiastic about spitting at anything that caught their attention. They could easily shoot a powerful stream at an object 2-3' away and they were very accurate, particularly if something was moving. It seemed that any animal that moved about was likely to be frequently getting hit with water.
With small frogs and insects that would almost certainly mean getting knocked into the water and being eaten (the archers are really aggressive eaters and will frequently lunge and bite at things that were too large for them to swallow). But even with larger animals I thought it would probably be quite annoying and distressing to be constantly getting spat at.
I personally agree with hydrophyte's suggestion - do a nice planting job on the emersed portion of the tank, include some interesting plants and the archers by themselves should provide sufficient activity and entertainment. You can periodically release some insects, which can serve as temporary inhabitants
Hope that helps.