Only you can decide.
As far as I know, there is no reason why the jab should be any better/worse than previous ones.
In my experience, diabetes has not made a difference to the reaction to the vaccine - my non-diabetic partner had far worse reaction to the jabs than I did.
I have heard that different people react differently to the different vaccines. It may have been that you had the Astra Zeneca for the first 3 jabs and then Moderna for the 4th.
My understanding (although I am definitely not a virologist) is that the immunity from a covid virus (including a common cold) does not last long which is why we get further boosters. I would not expect to still have much immunity left from a Covid vaccine given a year ago.
There has been a lot of press coverage about the latest prevalent variant. Any illness makes diabetes harder to manage so I too have my jab booked for start of October.