I'm bad at math so please forgive me if this is a dumb question. My usual game is 1/2 with a $4 bring in. When trying to manipulate SPR w preflop raise sizing, should I default to thinking that I'm playing 1/4?

Good question.

First of all, the stake (1/2 vs 1/4) doesn't really matter after the preflop raise. The SPR has nothing to do with stakes. It's just the ratio of effective stacks to the size of the pot.

For instance, a $10 raise at $2/5NL with $300 effective stack sizes yields the same SPR as a $10 raise at $1/2NL with $300 effective stack sizes. In both spots, (assuming it's heads up), the the SPR is $300:$20, giving us an SPR of 15.

One thing to consider, however, is that prior to the $4 bring in, the game is really still $1/2, since an opening raise is attacking $1 and $2 blinds.
In other words, if it's folded to you preflop, and no one has limped in, your raise sizing should be relative to $1 and $2 blinds. But once someone has brought in the $4, the game is $1/4, and your raise sizing should be larger.

Everything that DaYu said is right, and it's good to know.
But to be honest, SPR is only one of a few tools that we need to use when we are making decisions.

When we are playing, we need to strongly consider what our opponents are thinking about, and how they are adjusting. We need to know how much they are willing to put in money with worse hands.
If we have villains that will call 3 pots sized bets with top pair decent kicker, then stacking off with an SPR of 13 can be fine.
But if they will fold the turn unimproved to a half post sized bet with top pair decent kicker, then trying to get someone to stack off with and SPR of 5-8 even can be a problem.

In addition, we need to try to understand if our villains are thinking about money in terms of size of the pot or size of the bet. Are they looking at your $100 bet and thinking, sh*t that's 1/4 of my rent for the month, or are they thinking $100, that's 1/5 the size of the pot, I've only got to be good here 1 in 7 times to be good here, he's bluffing X %, blah blah blah.

And none of this is ever going to be quantified by the size of the blinds or opening raise.