Halcyon, it's not whether the fly line is heavy enough, but rather whether the rod can handle the larger trout in that lake and not overtire them before releasing.
A 5 wt. line has virtually no effect on the breaking point (your tippet would break first), but rather it would be like adding more weight to your spinning outfit. The 5 wt. line might help you cast farther, but it will be less delicate and possibly overpower the rod if it is too heavy. If you can cast with the 4 wt. line, there would be no need to go heavier. (I happen to have my lines 1 line weight heavier to compensate for my poor casting form. Sort of a permanent Band-Aid solution).
As for leader tippet, might I suggest some 6 lb. fluorocarbon; it's less visible underwater so you can go heavier. If you go much heavier, you might not be able to thread it through the eye of some of your smaller flies.
Have a great trip!