Hi, welcome to BYC!
The feed is fine. Lettuce doesn't have much nutrients and my free range birds don't care too much for it either. But I still put out the extra. Other scraps are fine such as watermellon or cantalope rinds, broccoli, peas, cucumber, squash, strawberry tops, tomato tops... even chicken and eggs.
Note since my birds free range and have access to the garden, I have no true control over what they eat. And I put my compost out to their area every day. And some things like their access to peaches, apples and plums are seasonal. Treats should be kept to less than 10% of their total daily intake. I don't count my garden items in that, again because it's part of their range.
But as 21 hens says, balance is key.
At 4 months I would probably make oyster shell available on the side. Which is actually all I do anyways. Since I have all ages and genders in my flock I feed a flock raiser with 20% protein (for chicks) and OS on the side for layers. If you switch to layer... just make sure you don't diminish your protein level with the garden and scraps as the 16% in layer is the minimum suggested to maintain good health in layers. If you provide OS on the side... you can feed any unmedicated starter, or flock raiser that you like if you don't want to switch to layer. The only thing layer has that others don't is the OS mixed in usually at a rate of about 4-5%.
It's good that you are hesitant to throw out
extra "treats"!
If more people were like that they would have less flock behavior and health issues.
Note *sometimes* new layers will lay a soft egg or they may even practice in the box but not lay an egg the first time or so. A soft egg does not have to mean lack of calcium and often doesn't. Layers just coming in or out of lay will sometimes have a softee just as part of the system getting up and running and working out the hiccups. A hen will usually use up their own calcium before you see soft eggs for that reason. Sometimes the softee will be laid out in the yard or while they are sleeping. It isn't a reason to panic. OS doesn't go directly to eggs anyways. It is absorbed by the hen into the keel bone which then distributes it to the shell gland.
If you do switch to layer once you start getting eggs, the birds won't go through that much OS. When I used layer a 50# bag of OS lasted me more than 2 years with only 3 birds. So in that instance it might be worth it to buy the small bag. Now my flock has at least 15-30 layers depending on season and a 50# bag lasted not quite a year but is much better price than small bags and I need it available at all times since I don't use layer. Every situation is different, and there is no perfect way. Make the best decision you can and switch it up if you find it isn't working.
Oh and I should mention.. if you have time to consider fermenting your feed, it does save a little cash (10% for me), increases nutrient absorption... and a biggie, decreases the nastiness of the poo smell!
It's as simple as mixing feed and water, stirring once a day and serving out after about 3 days. Check the link in my signature line and feel free to ask questions. Highly recommended!
I've been doing it about 1 year and never had to throw any out for mold (peoples biggest fear).
You must be getting exited for your first egg to be coming soon! Waiting is so hard.
I'm just teasing! We worked really hard to move and finally made it to the north coast... almost 25 years later. Try to stay cool. 
