1/5 bar of 4g is better than 5/5 3g. So your signal strength/quality has improved slightly.
I am not a tech-expert in that field, but that is my understanding.
Depends entirely on what you are doing - and could be quite different for voice or data.
The number of bars relates to signal strength - and a better signal will mean:
- potentially lower power needed to transmit / receive (so longer battery life between charges)
- less likelihood of drop outs (loss of calls, lost / resent data).
3G vs 4G, ignoring technicalities of what various telcos call "4G" can probably just be considered as 4G offering higher maximum possible data rate.
So, what does this mean for you? For voice calls, any higher rate than necessary is not of much use. Lower power use and more reliable connection of more bars on 3G is probably better.
For data use, not such an easy answer. If you don't need much sustained throughput, a more reliable 3G might be better (and give longer battery life). If moving a lot of data en mass, a faster 4G rate would be better, providing that drop outs, retries etc. caused by a poor connection don't reduce the effective rate below what a more stable 3G connection can achieve. It will probably depend on why the "bars" are low for 4G - is it something constant (you are just on the edge of the covered area), but the connection is reliable and stable, or does it fluctuate. Another factor, is how many others are using data via that cell. No point being on a 4G that is being flogged by others, if a 3G connection can give better speed because no-one else is using it.
There is no definitive "which is best", but my off-the-cuff would be, better signal on 3G for voice, and probably 4G for data.