1/ i am hoping to build it on a patio.....concrete paving slabs ok as a base for the pond?
Slab is a term for a poured patio. Concrete paver is a term for small piece of pre-made concrete, like 12"x12". Not sure which you mean. The weight of water in your pond would be about 6 tons. That should be OK for a slab, but cracks are certainly possible. Kind of depends on how well the slab was made.
On pavers isn't exactly a problem. Yeah there can certainly be some settling, but that's true for pavers even in a normal application. It's not unusual to have to pull up pavers every 5, 10, 15, 20 years and relay.
2/ we are looking into the best/cheapest way to construct the walls....suggestions and experiences please?
3/any other advice greatly appreciated
Best and cheapest is concrete block filled with concrete and rebar. You use a block that is called a "bond beam". Those allow you to place horizontal strands of rebar.
I have built with wood and block to see the differences. Pond in the back is wood obviously, pond in the foreground is concrete. I also torn down a wood pond after several years to see how it held up.
Cost is about the same. Labor is a lot more with wood which surprised me. For lifespan it wasn't even close, concrete (when built correctly) was no question better. Today I wouldn't consider wood except as a veneer for looks.
4' high is serious height. Some people nail together landscape timbers and that works really well for say 2' (lifespan aside). For 3' it starts to get dicey, depends on pond size. 4' really gets dicey. 6 tons is a lot of water. If failure happens it could do a lot of damage maybe hurt someone. 6'x4' is pretty small, wood could handle that, even 4' high, but you need serious corner structure. But there's still the lifespan issue. Sitting next to a 10 year old wood pond is not what I would call safe.
On a slab the slab is the foundation. For pavers the "best" way is to pull up pavers and dig a proper footer and then block on top of that. Local building department can tell you how deep and size of footers, just use the same as for a house. For a pond in FL the pond bottom can be the same level as the footer. That gets you some of the water below soil which will help even out water temp which is a good plus.
Concrete block is easy because you can just dry stack it, no mortar. Add rebar and fill with concrete. Not a lot of skill is needed. The horizontal rebar in the corner is super important to get right, steel must be contiguous in the corners.
