if you chrage a 4Ahr battery with a 1 amp current it takes 4 hours for a full charge. Alternatively if you charge with a 4 amp current it takes 1 hour theoretically. Some manufactures do not recommend booster charging as the internal plates may get damaged. (I have some experience on this). However, looking at the data you have given, I do not understand how you are going to charge a battery of 6V/4ahr with an out put of 100mA
Nevertheless,you may obtain an unregulated low-voltage dc supply using transformer-rectifier arrangement.Rectifier diodes could be connected in bi-phase arrangement or most common full wave ckt configuration based on four diode bridge. The following formulas apply:
Bi-phase
Output voltage(Vdc) 0.71 x Vac
Output Current (Idc) Iac
Input voltage (Vac) 1.41 x Vdc
Input current (Iac) Idc
Reservoir Cap.
recommended min 2200 x Idc
(micro F. if Idc in A)
Bridge
Vdc-1.41 XVac
Idc-0.62 X Iac
Vac-0.71xVdc
Iac-1.61XIdc
Res.Cap. 2200 x dc
In each case the reservoir capacitor should be rated at around 2 times of Vdc in order to provide with adequate safty margin.
You may use fixed voltage regulators to get the out put. 7805 & 7824 with 1amp out put current are recommended. Care should be taken to ensure that the inregulated dc input voltage is within the range specified by the manufactures (e.g +7V to +25V max for a 7805) Where the input voltage is very much higher than the output voltage, power dissipation within the regulator may be excessive. Conversely, where the input voltage is only marginally greater than the output voltage, regulation may be very poor and ripple may be present at high current levels. The optimum input voltage (on load) must normally be at least 3V greater than the regulator's out put voltage. In order to avoid excessive power dissipation, the input voltage (off load) should normally be no more than about 6V greater than the output voltage.
Cheers !