I would hold out a month or so and check back on the discover pre-qual. I think you are close to the unsecured.
@Anonymous, congratulations on the new card. 
The good news is that your new card will probably get a small CLI at the six-month point. When it becomes three months old, you can start requesting CLIs once a month as long as you don't get your hopes up. People used to report being able to get an additional CLI in that three- to six-month range, but that seems to be happening less often lately. If you do see success, that wouldn't affect your six-month CLI. However, starting with the six-month CLI, you won't see limit increase offers any more often than once every six months.
The bad news is that this card probably won't grow my much, although there have been exceptions. Capital One cards issued with very low limits tend to be stuck in "starter buckets," and limits likely won't grow past $750 or so. If you happen to be one of the lucky ones whose limit increases to $1,000 or more, the chance that the card will continue to grow improves significantly. The general recipe for increasing Capital One limits is to spend and pay in full. However, as mentioned, spending may not help much on a starter card.
Start checking the upgrade link when your card is about three months old. A lot of people see offers to upgrade to a full Quicksilver at about the four-month point. The chances of this happening sooner or later are excellent. Usage habits don't seem to contribute a lot when it comes to generating a Platinum-to-Quicksilver upgrade offer. If you hit four or five months and you're not seeing an upgrade offer via the link, call in or chat. Sometimes, the link misses things.
With the new Platinum, use the card as much as you'd like as long as you never exceed the limit. Pay as often as you need to in order to keep credit available. Curtail use on this card as the statement approaches. You want to the statement to cut with a balance between $5 and $86 (28.9% of the card's limit).
I'd suggest using the secured card once a month then paying that balance as soon as the charge posts. The goal would be to show usage and to generate a statement balance of zero. If you want to use the card more, that's fine too. Just make it a goal that the statement balance is zero. If the statement dates on your two cards are staggered, you can use this card for charges while your Platinum is "resting."
The reason for controlling your balances in this fashion is so the utilization portion of your score will be optimized in case the opportunity for a new card comes up. FICO likes it when one card reports a small balance with the rest of one's cards reporting zero. Captial One cards report the statement balances soon after the statement is issued.
If your best laid plans for controlling your statement balances mess up from time to time, it's not a big deal. Report your desired balances the next month, and your points will come back. Once your score grows, you won't need to be so meticulous about your balances. What you're seeking right now is a good "look" in case you want to apply for something.
Definitely keep checking the Discover pre-qual tool. Also, give your credit union a call. They might surprise you. You have room for two more cards over the next few months. Card number five should wait until your secured card reaches 25 months in age.
With Discover and the credit union, I don't think timing is all that critical, i.e. I don't think waiting to act on an opportunity is a big deal. With some banks, we'd suggesting waiting three months, six months, or whatever since your last app. Capital One is one of those banks, but you took care of them by applying for them first.