davidsai,
as 1.1 V per cell is considered discharged, you won't be able to get a 5 cell battery to drain to 4.8 v. On your 4 cell pack, 4.4 v is considered discharged. On your 5 cell packs, 5.5 v is considered discharged.

Septic,
i wouldn't trust a MM with my batteries as they don't typically load the system. You need to load it with 300 or so ma to see what the battery is going to do under a load--which is how you use it! Static batteries can have plenty of volatage. However, place a load on it, and it could go south--you don't want to find out the hard way! Get a hobbico rx battery tester--it will have the connector that you need already attached. Simply plug the battery in and see what it says!
Here is a nice digital one:

[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXWW16&P=7]digital[/link]

Or, here is a cheaper, analog one that will do the same thing:

[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXNK81&P=7]analog[/link]

apparently, they do not come with a battery connector anymore (like mine did). I think you'll also have to order this connector for it:

[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXL332&P=M]futaba[/link]

[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXL335&P=M]JR[/link]