[FrankHudson]

The usual response on the 4 string flat top "acoustic bass guitar" models has arrived, and I endorse that outlook as generally valid. Unless you're just fooling around for personal pleasure or recording into an interface, you're going to need an amp with this or the other options. I agree the Fender Rumble amps seem to work well and are very lightweight. If weight or size isn't an issue, older used bass amps are legion too.

I've used an U-Bass, and if you want something fun that sounds a bit like the sound of an upright bass and you don't want to look out of place at a house acoustic guitar jam or the like, there's a great deal to be said for these.

If you want to learn to "play bass" and sound like bass on records, there's a lot to be said for just getting an electric bass. Those inexpensive Hofner "Beatle Bass" models Steve mentions upthread are lightweight and for many boomers have a nice "Be Like Paul" vibe.

Here's one reasonably priced electric bass I happen to like: the Epiphone Jack Casaday model. I happen to like Casaday's playing, but this model with its special variable impedance pickup and large hollowbody has a lot of acoustic guitar vibes even if you've never seen acoustic Hot Tuna. I string mine with flats.

If playing acoustically in technique and "vibe" is what you're after, you could consider a baritone acoustic guitar. I don't have experience or offhand knowledge on what models are out there currently, but I have played baritone electric a little. You can play chords, pick and finger with the same technique you use on regular guitar, but you're in a lower register.