That is right. With premium program you can earn up to 5.10% (+1.02%)* annual interest.
The method to raise your interest from 4.08% is by investing in the FUN tokens? I am currently running my coins on the base interest of 4.08% and any boosted amount is always welcome. I thought the FUN token investment was a separate scheme altogether, meaning that two investment schemes were running on the site side by side - correct my if I was mistaken.
Yes. You have to check premium program spreadsheet, at premium tab, containing the tiers of FUN token investment and how much they boost your BTC yearly interest income. Keep in mind in every tiers you have to hold the tokens for 1 year before having access to the full benefits of the investment.
Even though the maximum tier (500,000 FUN tokens) guarantee you the extra +1.02% on BTC interest after a year, it worths mentioning the best cost benefit tier is the 12,500 FUN tokens one, guaranting you a 10% boost after 1 year (about +0.408%), against a 25% boost from the top tier.
For me the tier with holding 12500 tokens is the sweetspot. You get up to 8 WOF spins, get some cashback and you get extra interest on your BTC. I am not sure if going for the top tier premium membership is worth it, but thabks to your investment and topic, we can follow the progress and see how long your ROI is.