How many total accounts do you have on your credit report and what is the average age of them? You say your credit history is 4 years old, so I'm assuming 4 years is your oldest credit line meaning that your AAoA (average age of accounts) is less. If you only have a couple of accounts, you are considered to have a "thin" file which can hold people back from larger limits depending on the creditor.
As far as utilization which you briefly touched on, your monthly spend relative to your limit is irrelevant to scoring and your overall file. What does matter is what type of balance you let report relative to that limit.
You can spend $1000/mo on a $1000 limit as long as you pay it off or pay it down significantly. Actually, you can spend more than this if you make multiple payments. Spend $1000, pay it off, spend another $1000, pay it off and let $0 report. You just spent $2000 but reported $0 for this cycle in this example.
Are you letting a balance report? When you say 30%, do you mean you just spend $300/mo on a $1000 credit line or are you letting a $300 balance report at the end of your credit cycle? These are two very different things. Spend doesn't matter so long as it gets paid off. Balance left is what impacts lending decisions such as approvals, approval amounts, CLI amounts, etc.
If you aren't already doing it, you want to make sure you're letting less than 9% of your credit limit report. So, on a $1000 limit you would want $80-something to report at most; never $90+ or it will look less favorable to lenders, especially if you are applying for new credit.
