At 5 units and above, you get into commercial lending. Now we need a completely different type of appraisal. The value of the property is now dependent on the income that the property produces and the expenses the property carries. When the income is low, the usual remedy is to increase the downpayment needed by the buyer. It's not uncommon for smaller buildings to need a 35% down payment, for example. It's also not uncommon for a 5-plex to be across from a 4-plex, but have the same asking price, simply because of the way each property gets its valuation. For that reason, 5-plexes can be a very good deal for the savvy investor.
As far as specific lenders, feel free to give me a ring, and I'll give you the names of some people I've worked with. I'm asking you to call or email me, because I'm not keen on posting them here.