Can someone explain the difference between these two tires? I just had to purchase a new tire to replace one that had a sidewall puncture. My E320 has 960 miles on it and it had the MXV4. I only found the MXV4 Plus locally. Is there a problem mixing the two?

I don't know the exact answer, but if the thread pattern or other characteristics (i.e. threadwear #, 340 Vs 400) is different then you can expect the two tires to behave differently under different conditions (i.e. braking). You don't want three tires doing one thing while the other one doing a different thing. How much difference is there in their characteristics and behaviour? I don't know, but I would not mix tires if I was making the decision.
Also I believe they have different prices.

my $0.02

__________________
'02 C240
'00 LR Discovery
'72 280SE
'67 280SL

---past---

'79 280E sold (RIP)
'86 300E sold
'87 300SDL sold
'90 Laforza sold
'95 320SE sold
'98 ML 320 (sold)

The MXV4 Plus is the the replacement for the MXV4. I do not think the standard MXV4 is available anymore, I have not seen them anywhere, only the MXV4 plus.

On a sidenote, I use the MXV4 Plus on my car, and let me say that it is the best tire I have ever had the pleasure of driving on.

__________________
Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record