Yeah 4x slicken or chill fog per encounter is a bit much, but at the same time I think having something per encounter is good, which is why I think making it something 1 per encounter, plus 4 more per rest, would be a better balance. Perhaps up it to 2 per encounter at even higher levels or with a talent.
Even better I think would be regenerating 1 per encounter, that way if you go a few fights without using them you can end up with a full load again or you can just use one per battle and immediately get it back.
Not sure how I feel about regenerating spells, but I definitely like the idea of 1 or 2 encounter spells better than the current system. Once I get my wizard or druid to a per encounter basis all but the toughest fights seem to follow the exact same pattern: I lay down a chill fog and then spam fan of flames with my wizard, and queue up 4-5 charges of dancing bolts (or sunbeam if there's a fire weakness) with my druid. Everyone else just needs to stand still while everything around them dies.
I really like the idea of having 1 or 2 per encounter spells so at higher levels I can still lay down a chill fog and maybe a slicken or fan of flames at the beginning of every fight for free, but any additional spell require I pull from my per-rest pool. Perhaps add in a talent that allows you to specify a single spell be completely per encounter use, so if you do actually want your caster to spam your favourite spell you can build him around it.