This is a great move by Coinbase to make "owning more crypto" easier for the average (accredited) investor. But 4 coins != the market.
Like others, we've tried to maintain our own index of top cryptocurrencies, and experienced all the same repetitive issues of calculating asset shares, etc.
So we created a tool to help ourselves, and hopefully others: https://thecoindexer.com. It takes an input portfolio (on CoinFYI or others) and periodically calculates and emails the optimal* rebalancing trades to match a target index. Current indexes are based off CoinMarketCap's top 5, 10 and 25 assets.
We've still got a lot of kinks to work out, but feedback/suggestions are welcome.
*optimal = fewest # of trades on preferred exchanges