4K for an Iso Booth????? You can definately build your own booth for under a thousand. I ordered plans from dawbox.com and my booth has worked like a charm to cut out all of the noise from trucks, loud speakers and that kachoonck kachoonck B.S. which I can hear at all hours of the day or night from the nearby highway. I would recommend building a booth with a layer of sheet block underneath the foam. You could do the whole floating room thing, however it is pretty involved. I helped a buddy build a studio over the last several months and you have to figure out all of the dynamics of resiliant channels, double sheetrock glued together with a layer of sheet block, what will your room be floating on, ect. The floating room thing is not exactly portable if you happen to move. With the vocal booth, you would need to silicone the seam in order to have the desired isolotation, but at least you could break down the booth and re-build it if you needed to. One more thing........The vocal booth plans I ordered come with all the instructions you need to build a plenum, in other words the ventilation.
F.Y.I. The biggest soundleak that you have right now is the window/s. Cover them up with MDF. You can prevent it from being an eye sore by putting a thick fabric over the top which will also help reduce the noise. Many fabric stores sell partial pieces or scraps. If you go with a velvet or thick textured fabric it will look and sound great.
Hope this helps.
Good Luck!
