I have a similar problem with 4X spike at 0.68 ips but generally in the horizontal readings. I got some 0.5X harmonics as well, 4X, 4.5X, 5X and 5.5X at lower amplitudes.My gearbox is a simple speed increaser, single. What I've noticed is that 4X has always been a prominent spike in readings whether the overall was 0.1 ips or 0.8 ips. I suspect its an inherent frequency. There are no components rotating at 4 times the incput or output speeds to my knowledge. It is highly unlikely the bearings would produce a frequency at exactly 4X but I have unclear bearing data to verify this for sure. I somewhat agree with Greg Locock on the reasonance theory. I started a thread on my problem. I invite you to check it out.