SSC

Well, so far so good, but! If I may, I suggest that you space out your variations a little more. You don't need to run through the entire piece even if you start fast. Though I don't necessarily mean tempo contrasts, note durations and the general rhythm can be variable.

Thing is, what you're doing is spilling all sorts of musical ideas into the score, when you have quite a bunch just with the first 10 measures. Remember the exercises we did with variations?

Another important thing is, don't worry about length now. You're not writing with a specific classical form and it's OK to start with short pieces. The deal is, writing short things is not necessarily easy just because the duration is short. Take for example lieder from Schumann or Schubert, which are generally short. They're very compact and packed full of very complicated harmony and such things, yet very short.

There's a thing about short pieces which you have to consider. It's not that "the goal" is to write something long, but instead the "goal" is to write something that fits what you want to do and the amount of material you invested. You can build an entire symphony out of a 4 note motive, likewise, you can do a 1 minute piece with 800 different motives.

And another thing is that the shorter the duration, the more effective the usage of your material needs to be. You don't have time to develop or linger on repetitions, so everything must be pretty snappy and coherent on the first listen (though this can vary, if your intention is otherwise.)

Now, 4 minutes isn't long exactly, but you need to plan what you do a little more. Generally, a lot of pieces have repetitions, and such other things that add to the length. But they're not without reason.

So, let's go a little over planning.

Since you have your first "theme" there, which I think is pretty nice and catchy, you should probably return to it soon. Changing "key" isn't so important, as it is actually coming back to it. I think more interesting than changing key, is a different harmonization perhaps.

So, you can split the piece into sections where, for example, your main theme shows up. Unless you introduce another one, that is. This works as sort of setting up "check points" along a duration of time. For example, say you have 5 variations of your theme, which you want to go through before the end. You have a measure now as to how to space your piece so that all 5 show up before the end. Plus add in different bridge/contrast sections, and you got something of a plan already.

But that's just one idea, there's no real "right" way to do it, but setting checkpoints for yourself is a good strategy if you have a specific duration in mind you want to shoot for. You also could follow one of the classical simple forms in some way or another, such as ABA form, though loosely. Remember, using material from other periods is fine and dandy, you don't necessarily need to use them just as they were back then either. That's the whole gist of neoromantic/postmodern/etc styles. Just take what you find useful and do what you want with it.

Also, another thing that may help is not writing the piece from "start to finish" literally, but jumping around. Write your ending, write your middle section, jump to whatever you think you have ideas for. It's of course a different approach with its own challenges, but sometimes it's a pretty good method if you're stuck writing "start to finish" but have ideas for what comes after. Also where the checkpoint method is useful since you can coordinate your writing with the checkpoints so you can control the coherent "whole" of the piece even if you're jumping around.

Back to the duration, don't put so much emphasis on how long a piece is. It's good if you have a set duration in mind, but changing your mind isn't illegal. Remember also that many classical forms are sectional, as in you have movements. Though, you may or may not want movements, but it's something to consider. You can maybe do four 1 minute pieces, for example! Doesn't mean they have to be entirely different, but it may help organize your ideas better while keeping your overall "duration" for the entire work within your goal limit.

To what you have, like I said, try to take material from your beginning as basis for variations, etc that come later. You don't HAVE to, but then again your variations can be very contrasting so as to not even seem like variations. It's up to you really. Though, I'm personally inclined to have a piece have a certain sense of "consequence", if you use a specific motive construction at the beginning, you probably should come back to it at some point even if it isn't a direct repetition or right out obvious that you did.

Also, there's the question of having a specific "climax" to the piece, and where to put it. But that's really something you have to define and see for yourself, since it can be an infinity of different things.

So, I hope this helps you organize your ideas better and continue to write it.