What is active duty like between the ranks of E-1 and E-4?

SGT(P) (Join to see) Active duty is what you make of it. Be a squared away soldier. Work hard to master your MOS and the duties of a soldier. Create opportunities to take on increased responsibilities so that you can earn promotions.

go to the motorpool, cleaning out a connex millions of time, waiting around for hours for nothing. Work and pt. Field and range. Area beautification, lots of it. It sucks but some strange reason you love it.

Figuring yourself out because you have to grow up sometime. That's the time to nail down those good habits so they're second nature when more lives count directly on you. Other than that, try and remember you're not indestructible.

Who knows?
Actually a person's experience in the Army/military vary: duty station, MOS, their CoC, schedule for that year, etc. That doesn't even take into account the person's sex, age, ethnicity, work ethic, personality, emotions, health, marital status, kids, etc. In general lower ranking soldiers get hazed upon arrival for awhile and usually treated like crap. A soldier's experience greatly depends on how he or she acts and performs. There's no manual nor it is the same.
Do I regret joining? NEVER....As a Medic (68W) I enjoyed my job most of the time. Getting calls from the guys in the barracks to get an IV because they drank too much was frustrating. Good old Fort Drunk, NY.

Doing mundane tasks "just because". Cleaning just to clean.
But also learn as much as you can about your MOS. Square yourself away. Try to take on increased responsibilities. Don't let the mundane and constant cleaning get you down, let the whiners whine.

It's great. You're put in charge of things. Given oodles of responsibility. Weekends and Holidays off.

Slave labor, being treated like a child, unable to be blamed for anything, lots of wondering what is going on, very little responsibility....I think that about sums it up.

PVT to SPC : Execute orders from NCOs and Officers appointed above you. SPCs are supposed to be groomed for initiation into the NCO ranks.

CPL: an E4 tasked with increased leadership responsibilities. Capable of doing majority of the things a SGT can do but without the corresponding pay increase.

As others have said, lots of pointless "busy work", hurry up & wait, PT, cleaning... along with the opportunity to better yourself, both personally and professionally. As has been said, learn good habits - and don't forget to listen & learn from those with more experience in your field. Again, it is what you make of it. What you do in that time period determines much of how you will perform if/when you move on to higher rank(s).

Hours of inactivity, followed by busy work of limited significance.