Your schedule is a tough one, but if you have four days off in a row, there are lots of things you can do.
First off, if the old friends are only available on an occasional basis, make some new ones to fill in the gaps. You're not dumping anyone, but neither are you sitting around moping over the people you can't see.
Take a class, join a club, find a church, volunteer. Do something--anything constructive--to get you out of your four walls and into new areas of interest.
Look around your apartment. What could use some improvement? Maybe take a class in carpentry, upholstery or photography to liven things up a little.
Take a cooking class and see if you hit it off with any of the other people you meet there. Worst case, you expand your dinner menu and get to eat some delicious dishes.
Are you bilingual? If not, jump in. If you are, how does trilingual sound?
Join a yoga class where you can mingle with others who share one of your current interests.
Volunteer your time at the library, a local school, a nursing home, a food co-op.
This is just the tip of the iceberg. Go, looking to have a nice time, and if you get some friends out of the deal, so much the better.
Don't be too limited in who you would consider for a friend. Someone ten years older? Why not. They might have an interesting take on life that you would never have considered. A person from a completely different culture? It's good to stretch and learn new things. A musician or a teacher or a gardener? Cool. You don't want to hang out with only nurses, do you?
In short, there is a whole big world out there that you get to explore a couple of days a week. Don't worry that you won't know anyone when you begin. If you reach out, that will change.
One word of caution. Listen to your gut. If you hit it off with someone, meet at first in neutral places like a classroom or a restaurant to see if your comfort level is consistent. If someone gives off an ookey vibe, keep your guard up and don't put yourself in a vulnerable situation. Don't know if you have a land line, but if you do, give out only your cell number.
Other than those common sense precautions, throw yourself out there and see what you can find. Make it a point to spend at least four hours a week going on expeditions and doing new things.
I think you will find what you're looking for.