You cannot be sure the batteries are at the same charge level, so you should preferably charge the cells in parallel. Use roughly 1.4v and regulate current to cells individually (I've found documents suggesting 140mA per cell for a AA. That's only .21W per cell if you can charge them at 80% efficiency!) You can use a timer, as Nimh batteries aren't as sensitive or dangerous as Li-ion, but you would be better to use thermal protection and detect voltage dip from overcharge to stop the charging cycle. You can likely find a 1 or 2 or 4 cell NiMh charger that can operate at 7vDC. The advantage, if you'd like to build your own, would be to get a much better efficiency level, but it'll be quite a bit of effort.
If you really want to charge the cells in series, you should start with brand new, matched cells and always use them together for both charge and depletion.
To charge multiple cells in parallel:
Start by building or finding a step down switching voltage regulator capable of delivering 140mA for each cell you want to charge at a nice steady 1.4v
Switch this current through the cells to regulate it to 140mA, and during the off time of the switching cycle, measure the voltage on the cell. When it starts to drop or an overheat condition is detected, stop charging that cell. Because solar cells can vary widely in their output and you won't always have direct bright sunlight, it would be wise to also build in an efficient intermittent charging option. This would be an advantage over an off-the-shelf charger, which you can't predict the performance of during a "brown-out"