if you get another box turtle, you really are risking the health of both in a small environment such as this with turtles at close to adult size. 4 inches can be nearly full grown for a three-toed box turtle. If it was 4 inches a year and a half ago, it is likely old enough to sex by now, but photos would help us tell you (an inexperienced person with three-toeds is most likely not going to be able to do it).
Also, don't mix a male and a female in such a small space. The female would be hounded endlessly and become stressed. Male box turtles do not give up easily when they are in the mood.
I really would encourage you to wait until you can house them properly before considerring more turtles. An outdoor pen is best, and a very large outdoor pen with a group that has more females than males is the best way to have mixxed company.
Besides which, if you want to have a pair (bad idea, trio's are better, but again, only with enough room), I'm assuming it's because you want them to breed. A stressed female will NOT lay eggs, she could even develop eggs and retain them, becoming egg bound. Providing a nesting spot indoors is a pain in the butt, as is simulating nesting conditions. Also, without weather and seasons, box turtles will lose their sex drive and stop mating all together as they become out of sync with each other. Females don't have any particular drive to allow males to mate, males mate at odd times, eggs don't get developed, etc.
I breed both three-toeds and easterns and have no luck indoors. One guy I know has luck indoors, and has for a few years now, but still isn't sure if it's a good long term solution. He built a huge enclosure into a room with lighting and misting systems like a zoo exhibit. He's probably spent more on one setup for a group than I have on all my outdoor setups combined, and I am housing over 30 turtles in breeding groups spread over 7 pens.
Do not feel bad about them living outside. They do MUCH better outdoors and are MUCH healthier for it.
In any case, if you choose to ignore the experience of other keepers and get another, make sure you find someone who can verify the sex of your turtle. If it is female, get another female of nearly the same size. Quarantine the new female. Take both your old and new turtles to the vet with fecal samples for checks. Deworm them. Make sure the vet OK's putting them together. Pet stores are a nightmare of germ and parasite breeding grounds and any animal from them is a danger to others.
If you have a male... I would advise against getting a female, but I would also advise against a male in such a small space.
BTW, my hatchlings from '01 (now two years old) are around 3 inches on average and can sit on the palm of my hand without touching the edges or my fingers. Yours is likely not a really young animal.