Elliebee, I've been hypothyroid for 20 years. I cannot function unless TSH is at 1 or below.
My old GP recognised that the UK's system for assessing what is normal was different to the US, and that it's not just based on blood test but clinical assessment. I was always given levothyroxine to keep levels at 1 or just below 1. But it's different for everyone what they require. A level of 4 would be very hard for me to function. But to give you perspective, my very energetic friend is at 4. So there are other things going on and it has to be a clinical assessment as well as a blood test.
I hope that helps, there's quite a bit of literature out there on what I've said that will probably give better detail.
(I'll quickly add that the usual measurement system means that 0 or 1 is lower TSH than 4, and therefore higher levels of thyroid hormone circulating. So in my case 1 is good and 4 would not be good, but again it does depend on the person. Remember my friend is 4 and is not hypothyroid at all and well)