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I do know the younger you are, the faster you catch on. That doesn't mean you will not get there but may have something to do with why you are stuck. I started in court reporting school at 17 and was done in 2 1/2 years without practicing.
All I can suggest is keep practicing but maybe use tapes that are faster than what you are comfortable with to improve your speed. Try to write something for every word even if it is slop. Soon that will make a difference. Also, I would only concentrate on one-strokers for words that come up a lot. Don't clutter your mind right now with phrases that you will hardly ever hear. That could help too.
I'm inclined to think that if you're practicing four hours a day and not moving, then you need a big change. (But I am certainly not overly qualified on this subject). Try cutting your practice sessions down to 30 minutes at a time, no more than two hours day. It could be you made a mental block. If you can afford the money, pay for Realtime Coach. The cheapest one is $144 if you are a NCRA member.
How does it feel when you write at 140?