Replacing the batteries connected in parallel?
I finally manged to find the specifications of the 4 SLA batteries connected in parallel:

4X 4V0.7Ah SLA

please advice on a new SLA battery spec that could replace the 4 so that I could buy a new one that is more relaible?

 

4V is a weird voltage; only batteries with more than 2.8AH are available as a single battery. These will work electrically if you can fit them physically.

http://www.power-sonic.com/site/doc/prod/161.pdf
http://www.power-sonic.com/index.php?id=31&gr_id=36