A) If the compost is 'finished', the particles are very small and will be very water retentive - IMO, not well-suited physically for use in container soils in any appreciable volume. B) If the compost is NOT finished, it contains particles highly variable in size that break down very quickly into increasingly smaller particles. As unfinished compost breaks down, it does add some nutrients to the soil, but it also promotes the immobilization of N. When it breaks down to the 'finished' stage .... well, see A) above. Compost actually adds very little in the way of nutrients. What it does add can easily be supplied, either macro or micronutrients, by other either organic or chemical supplements. It increases water retention as it fills valuable soil pores and decreases aeration. The question I would beg of myself is: Why choose to use it, knowing there are other organic alternatives with better structural stability and that what it adds nutritionally can be replaced very simply with 1 or 2 fertilizer additives that will be required regardless of whether compost is used or not. If I had to use compost in a container soil, I would limit it's presence, along with peat, in my own bark-based soils to Al

In a container as large as that, the onus of convincing the plant to live up to its genetic potential lies with you. If you've selected a good soil and are able to favorably supply the other cultural needs of the plant - light, water, temperature, nutrients . . . there is no reason you shouldn't expect it to grow that large. Al

If all you have is 4 gal you can do it but it is true there is a trade off and it is certainly at the lower limits of feasibility. Big containers can't be moved or managed as well such as changing out soil, moving into protected environments or better season light conditions. Even the size is not the only factor. What is the shape and soil mix? I would rather have a potting mix in 4 gal than 5 gal of top soil. If the container is not high and vertical than the drainage characteristics of the soil may need to change. A low wide container my hold too much water while a high container will give the same soil better drainage. I had a black cherry in a 3 gal last year in part because I often lose track of what is what. I thought it was a Roma which I thought would give me some decent production out of a 3 gallon pot. It turns out it was a black cherry seedling with very reasonable production with 2, 6 foot vines. It really was a decent plant. Now the down side of the smaller container were quite rightly mentioned to be water and nutrient loss from run off. In a dry hot climate its going to work against you more. I am in a 5a zone which is not like LA or Vegas. I also went with self watering 5 gal container with potting soil. The self watering addresses both problems. Bottom watering does not wash away nutrients and I need to water less frequently.

Big as possible - 10 gallon minimum. And you will need a really good support system. I have one plant that if laid out flat would be 15 feet tall and is at least 4 feet in diameter. It is covered in clusters of fruit. The other 2 are very close to it in size. Inside, in a container, and under lights yours likely won't get that big but it will still be a substantial plant. ;) Dave