@NewDOOFUSfor20 the worked example from @TimeToParty is really helpful.
If you want to know the actual rate* for your particular pension, as the NHS is so big a lot of their own guides are online for all to see.
When you were in the 1995 scheme, your accrual rate was 1/80. So for every full year you worked, you would get 1/80th of your final salary. Just as explained above, that figure will be increased by a number similar to inflation - there is a very specific calculation already agreed on what that should be, they don’t make it up as they go along 
In your 2015 scheme, your actual rate was actually really improved. To offset some of the things that changed negatively. So now, your accrual rate is 1/54, so you get 1/54th of your “CARE” (career average related earnings) for every year you’ve worked. In your mind, it’s best to just imagine these are two completely separate pensions - and they’re most likely listed on your statement separately. But they’ll get paid together in the end.
For CARE, this is a bit simplified, but say you earned £30K in 2015-2016, and then had a promotion and earned £36K in 2017-2018. Your average salary would be £33K.
So, you “accrued” 4 years:
1/54th of £33K is £611 give or take the pennies.
Multiply that by your 4 years of service:
4 x £611 = £1644.
So when you retire (at age 65, according to 2015 scheme rules I think) you’d get £1644 per year - for those contributions from 2015-2018.
Of course, you’d also get whatever you accrued in the 1995 scheme, and more from the 2015 scheme after my hypothetical 2018.
It’s worth getting to grips with your statement... NHS pension is very good and I think you’ll be pleasantly surprised and reassured if you’ve contributed for 19 years!