Tanning does need to be toned down, especially with the time it takes to get one in TS4 and the result you get is far too strong.
They should keep it to a simple holiday tan.
If we consider current result 100%
I'd say 50-70% would be fine, perhaps with some randomization or two stages. I'd also like it if it took a bit longer to get. If they could do a gradual system I'd be overjoyed.
I also wouldn't mind it too much if sims got get tans outside of deliberately sunbathing to make it more meaningful to their look and what they've been wearing when they get it. Don't make a sim get it no matter what just because they've went out but maybe if they've been out for quite a while on a particularly sunny day.