Yes but not quite as simple as that see below. Also a 12V battery is more like 13.2V when charged. (2.2V per cell)
Except that the battery has only 4Ah of capacity.
To get 12v you are in series, so the current is drawn through one 6V battery then the other. Hence you will only get 4 Ah of capacity.
However, that capacity is stated as the 20 hour rate, which means that you can only expect 4Ah of capacity if you draw it out over 20 hours, so you would be able to get 0.2A for 20 hours. (0.2x20=4). (it might also be a ten hour rate, in whcih case alter the numbers)
The actual 1 hour discharge capacity will be considerably less than that. If you chose the discharge rate such that the battery lasted for an hour you may find that you could only draw say 3 amps. Conversely if you took 4 amps out of it then you may find you only got 40 minutes instead of the expected hour.
If you are getting an hour out of two 4Ah batteries in series then you are probably using a 55W bulb.
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