J.R.
Hi, Meriah. I recently got re-married, but I was a S. mom for almost 3 years. I also work full time. My ex-husband has no contact with my girls, and he gives me no financial help either, so it was all me, all the time. My girls were 6 and 3 when we separated, so I can totally relate to your frustration. I love my little girls with all my heart, and I was not always proud of my parenting style, but I was EXHAUSTED.
While I was S. my parents kept my girls overnight once/week, and that was a nice break for me. At least I had a chance to go grocery shopping or run errands without having to drag along 2 tired, cranky, bored kids. Not sure if that's an option for you, but if so, ASK.
I can tell you that it will get easier. Your kids won't be 4 and 3 forever, and as they get older, they will follow instructions better and they won't require as much direct supervision.
My girls are now 9 and 6, and life is much easier. I have spent a lot of extra time teaching them to make their own beds, do their own showers, brush their own teeth, tie their own shoes, buckle their own seatbelts, dress themselves, etc. Sure, it would have been faster/easier to just do it myself, but I considered the extra effort an investment, and it paid off. Those little things add up, and I have a lot more time for other things now. For example, I can fold laundry or pack lunches while they take their showers. (Not exactly the 'me time' that we all dream about, but at least I'm not up until midnight getting everything ready for the next day.)
Just recently I started leaving them alone for 30 minutes in the evenings while I take a walk. FREEDOM!! :)
If I may make a suggestion, I would start with cutting the routine of laying on their floor until they fall asleep. Yes, they'll throw a fit every night for the first week, but I bet it doesn't take more than a few weeks to completely break that habit...and you'll have a few extra minutes each night for something else.
Hang in there, and as another mom said, be consistent. Remember that the extra effort now will pay off, and things will get progressively easier.