Four ads seems reasonable, especially in mobile mode, where you just sort of flip through the posts quickly and only stop if something catches your eye. For Tumblr users, I don't see this being an issue at all (although i'm not sure how effective the ads will be), because people don't invest all their personal information in the site, and they haven't been screwed over by tumblr like we all have by FB, more than once. Their ads also tend to be (i said 'tend to be') better and more contextual than on other sites.
On FB, the ads are just idiotic and crazy intrusive. Rather than wonder why FB thinks I'd be interested in a given ad, I usually wonder how the advertising company has the money to buy the ads in the first place. They are amateur ads. On twitter, you get like a Mitt Romney ad in your feed, and it's just ridiculous.
Up to now, Tumblr ads have been pretty good, visually at least, and they tend to be contextual to the types of content people share on their dashboards. I hope they start turning a profit, and doing it in ways that don't sacrifice the best things about the site. I mean, it's not like they were going to make $40million or $100million selling themes. I do wish a site like Tumblr offered an ad-supported version and a subscription-based no-ads version. But I'm just a believer in paying for a service like Tumblr, as a hedge against secret policy changes, etc.