Quick question, if your bottom tier is 4" off the ground, and assuming your lower tier is your BK, how will you transfer to the fermenter? If you have a pump to accomplish this, and are planning something that is 6 feet in length, you might consider a signle tier system. I started off with 3 tiers with folding shelves and ended up with a BK, from which I couldn't transfer to the fermenter without a pump unless I removed the kettle from the lower tier and raised it up (sort of defeats the purpose of a stand).
My reasons for going with a 3 tier the first time were primarily based on space. My folding stand stored in a 22" x 22" space. The second time around, I freed up some space and went to a 72" long single tier stand (lifting buckets of water up to that 3rd tier is no picnic.
Just throwing this out there. There are definite benefits to going all gravity. But you need to build that lower tier high enough so you can use gravity all the way to the fermenter. Depending on batch size, this requires a good deal of height added and/or a pump.
Since 2007, I have tried it a few different ways. I have gone from this
and ended up with this.
which eventually became this.