Hayden Carter, Tyler Walker, Ollie Webber and Denver Hume have so far arrived at Fratton Park this month.
And the Blues’ chief executive revealed they continue to hunt for potential fresh additions as the clock ticks down.
He told The News: ‘We are still going, we have not stopped yet.
‘We have one or two things that are going on at the moment. They might be ones we might not be able to do in January, but can potentially do in the summer.
‘Some players you might be able to prise out for a fee at this moment in time, but they may be free again in the summer, when the market changes for that particular player.
‘It’s never straightforward. We know what we want to do going forward, the type of player we want to recruit, while always understanding you need a balance as well, of experience and youth coming.
‘We continue to work with that. Danny and myself, Tony (Brown), Roberto (Gagliardi) and Phil Boardman are in regular dialogue with the owners on different players.
‘Everybody knows what we are looking for, it’s not scattergun, it’s quite considered in everything we are trying to do.
‘Danny and the team are working so hard on different players, it’s a moving dialogue, let’s see where we get to.
‘Something may happen, something may not, but it will be for the right reasons.’
Space within Pompey’s playing budget has been created through the January exits of Ellis Harrison, John Marquis, Paul Downing, Miguel Azeez and Gassan Ahadme.
Lee Brown’s exit to AFC Wimbledon is imminent, while Alex Bass has joined Bradford on loan.
But Cullen is adamant the Blues will not be focusing on quick-fix signings.
He added: ‘What we don’t want to do in January is go out for a player on a short-term deal who is not going to impact us straight away.
‘The other type of player we might look at is going for a good long-term investment.
‘There are players who are available, but are they ready to go when you only have 12 weeks of the season left? Are you getting value for money in that respect?
‘So we have to be very, very considered in what we do and make sure not to do anything knee jerk.’
A message from the Editor, Mark Waldron
You can support our local team of expert Pompey writers by subscribing here for all the latest news from Fratton Park for 9p a day, thanks to our 30%-off transfer window deal.