I currently own 23 automatics. 18 of these are side opening but none of them are tantos; don't know how many (if any) of them could be purchased w/a tanto blade. One of the 18 is a vintage stiletto that I is a special case and is not comparable to any of the other autos.
Of the 17 other side openers, 11 have blades between 3.5-4":
Benchmade Auto Stryker
Boker Kalashnikov XL
Gerber Emerson Alliance
Hogue EX-A05
Kershaw Rogue
MicroTech Stitch
ProTech TR-3 X-1
SOG Spec Elite II
Spyderco Automony 2
Spyderco Citadel
Spyderco Valloton D/A Auto
The Rogue and Citadel are discontinued and difficult to find and, if found, will probably cost close to if not more than $300. The Stitch is still available but costs more than $300. The Valloton D/A is a custom knife and, if you can find one or get one converted by Vallaton will cost at least $300 or more.
So that leaves 7 others.
If I were to recommend only 1 from the remainder it would be the Spyderco Automony 2 which costs about $190 ($290 retail) NIB from any # of dealers. Great knife and great company to deal with. It also has a corrosion resistant 3.5" LC200N blade on it.
On the other hand, if you just want a 4" auto and don't plan to do much w/it, there's nothing really wrong w/the $40 Boker XL either (except it doesn't have a safety swtich, which I prefer to have on my autos). You'd expect 13CrMov8 for such a cheap knife but it comes in AUS-8 which is a perfectly respectable steel for normal use.
Between these 2, I'd recommend the SOG (SE-61) which can be had for about $105 (retail $175). It is slim and compact and a beautiful (to me) 4" AUS-8 polished blade. It's a great deal IMO.
The 4 others retail around $250-$275 and sell for around $170-240, well w/in your price range. I just bought the Hogue NIB for $170 (it retails for around $250). The Protech goes for $180 (retail $250) . The Gerber goes for about $190 (retail $240). And, the Auto Stryker goes for about $240 (retail $275). Their higher prices reflect better steel and manufacturing quality but are otherwise functionally equivalent to knives costing less.
I like ALL of these autos based on their individual features (which is why I bought them) and I would have no problem carrying any of them, if it were legal for me to do (which it is not, except in the privacy of my home).
Which you will like best depends on what you consider important in a knife. Take a look at these on the Net . Then try to narrow down what you like best in a knife and which of these (if any) meets those preferences.
Good luck!
OBTW, the Microtech Socom Elite Auto is on my "buy list" and will probably be the next auto that I buy.