I was given a pot of (flowering) roses which I actually like and would want to keep and plant in my garden.
In this tiny pot there are 4 plants; each one has only a few roots as a quick inspection showed.
What would I need to do to save these?
I would try to pot on each plant, cut off all the flowers and buds. Anything else? Is it worth the effort? Will they survive at all?
Yes, they have a good chance - as my Mum has proven multiple times. Because she was always very busy, she just put the entire root ball into the ground, no further fussing. Admittedly, she just couldn't stand the idea of throwing healthy plants on the compost just because they were done flowering (she never bought them, but was given them as a gift - instead of cut flowers), but in about nine out of ten cases they did well for a few years.
Obviously they never grew much taller and pruning sometimes resembled micro-surgery, but we didn't have any major problems. Some were a bit less frost hardy, but considering the fact that they were probably produced as "disposable", they lived a long, flowering life.
So absolutely, go ahead and plant them. If you can easily separate them, do so, it will give them a but more room. (I'm not sure whether I'd try to rip a root ball apart, but yours is loose, as you write.) Just remember that the current heat wave is probably a tough time for new plantings. If you do plant, water carefully and keep it shaded for a while. I would probably wait a bit longer until the next rainy period...
The secret to transplanting anything plantish: make sure the plant is well watered before removing from existing area; try to keep as many existing roots as possible, but will also work on some plants with out root, like sage; dig the hole large and deep enough to contain the entire root system; sprinkle bone meal on the entire bottom of the hole; pour water into the hole about 2/3 up; wait for the water to drain out, but if it doesn't drain out fairly quickly you are planting in very dense soil, which may not be a good thing; place the plant in the hole and cover the roots with soil; water to saturation from above every day for at least a week.
