To start, the effective tax rate is meaningles; RRSP is done with Marginal tax rates. Effective rates is just a random number SimpleTax generates for fun, I like looking at it but it doesn't mean much. You both have the same Marginal rate I bet (
https://www.taxtips.ca/ - look up your province here)
Most provinces have a tax bracket around $92k. I use that as a quickie reference point (or $90k for simplicity)
Basic Assumptions
1) I assume in your retirement you plan on living on income under $92k (way under)
3) I assume you're young, at 65 its irrelevant. Income splitting exists which negates the benfits of spousal contributions.
Compare your respective total RRSP amounts; I bet yours is bigger!
Spousal RRSP's are only for couples that need to make their accounts equal. Its that simple. He should only contribute money towards the smaller RRSP (his own I assume since he has so much space). As to the amount, only enough to bring you down below the $92k bracket (presuming he's way over $100k). What you want to see is his RRSP room shrinking, not growing, with a projection of it dissapearing in the future. RRSP room isn't a race, it's a process. Its about getting large refunds now and paying small amounts later.
This does raise the obvious question; should you be doing spousal contributions towards him? That $4k might be better in his accounts to equalize the withdrawals before you turn 65.
the less obvious answer is it doesn't matter since the marginal rate upon withdrawal for each of you could be the same. This happens if you have $600,000 and he has $400,000. Some people think matching amounts mean the accounts are equal. In math terms they're equal if the withdrawals are in the same marginal tax band. do the math yourself for fun, would a couple with $600k and $400k withdrwaing 4%/year pay more/less tax than a couple with $500k each withdrawing 4%?
In both cases the couples have $40,000 to live off, is the tax bill different? Try $300k and $700k next, that's a good way to hammer home the idea behind marginal rates.