If you enjoy the forum please consider supporting it by signing up for a NES Membership The benefits pay for the membership many times over.
I was wondering if I could use the in 38 specials with 158 gr lead over 4.3 grs. of Unique?
Definitely not.
So-called "magnum" primers are designed to be used, and are rated to be used, only with specific powders (and then only within the range of charges specified by the powder manufacturer). Because they burn longer and hotter than "regular" primers, magnum primers will change, perhaps drastically, the peak chamber pressure curve of loads with powders intended to be used with "regular" powders.
Let me say again, with emphasis: it is not a question of which caliber; it is a question of which powder.
In my experience, primers store well for long periods. Just hold onto your magnum primers until you start experimenting with powders for which they are specified, which you will likely eventually do with a .357, and in the meantime go out an buy some regular primers for your Unique loads.