Don't worry! I'm going to explain how to modify a 2 meter antenna for 4 meters operations.
You need a 5/8 lambda for 2 meters band. Almost any mobile 2 meters 5/8 antenna is built as shown in the following example:
Choose an antenna built in this way (to be sure, verify that hot wire and the braid of the coax are not in short-circuit).
This antenna is more than 1 meter long whip and a base coil with few turns. The 1/4 lambda in 70 MHz is 1.02 meters, so THIS ANTENNA MAY BE MODIFIED FOR 70 MHz. In order to do this, cut the whip until SWR dips at 70 MHz. Is very simple, and works for ANY 5/8 antenna for 2 meters band.
For the mod, I choose a well-built antenna; a Caletti Atlas, built by the Italian farm Caletti. I buy this antenna from a surplus reseller, is new in box and shipped with base, cable and connector for 7.50 Euro, a real bargain for a quality antenna!
Resonance (3/4 lambda) between 159.9-161.5 MHz
(SWR < 1.1/1)
Resonance: (1/4 lambda) 55.770 MHz
in the ham bands:
50 MHz 1.9/1
51 MHz 1.8/1
70 MHz 2.0/1
145 MHz 1.9/1
If you not are a fan of SWR 1/1, this antenna may be usable on 6, 4 and 2 meter with no mod!!!!
But my idea is to build a 4 meter whip, and then began to cut the whip measuring SWR with the MFJ259B. I stop at SWR 1.3. The antenna is:
Lenght: 85 cm
(from the end of the spring to the end of the whip)
Resonamnce (1/4 lambda): 70.200 MHz
SWR 1.3/1 R=43, X=13